Add better exception handling.

Former-commit-id: f6ba09bdf9b16351b639490de55c5a0402acd7cc
Former-commit-id: 676fecf193f635d19f74ca3de41b8ced7e667352
Former-commit-id: 00484e51faae841544bdc27a2f59d227c664012c

src/ImageProcessor/Common/Exceptions/ImageProcessingException.cs

@@ -0,0 +1,44 @@
+// <copyright file="ImageProcessingException.cs" company="James Jackson-South">
+// Copyright (c) James Jackson-South and contributors.
+// Licensed under the Apache License, Version 2.0.
+// </copyright>
+
+namespace ImageProcessor
+{
+    using System;
+
+    /// <summary>
+    /// The exception that is thrown when an error occurs when applying a process to an image.
+    /// </summary>
+    public class ImageProcessingException : Exception
+    {
+        /// <summary>
+        /// Initializes a new instance of the <see cref="ImageProcessingException"/> class.
+        /// </summary>
+        public ImageProcessingException()
+        {
+        }
+
+        /// <summary>
+        /// Initializes a new instance of the <see cref="ImageProcessingException"/> class with the name of the
+        /// parameter that causes this exception.
+        /// </summary>
+        /// <param name="errorMessage">The error message that explains the reason for this exception.</param>
+        public ImageProcessingException(string errorMessage)
+            : base(errorMessage)
+        {
+        }
+
+        /// <summary>
+        /// Initializes a new instance of the <see cref="ImageProcessingException"/> class with a specified
+        /// error message and the exception that is the cause of this exception.
+        /// </summary>
+        /// <param name="errorMessage">The error message that explains the reason for this exception.</param>
+        /// <param name="innerException">The exception that is the cause of the current exception, or a null reference (Nothing in Visual Basic)
+        /// if no inner exception is specified.</param>
+        public ImageProcessingException(string errorMessage, Exception innerException)
+            : base(errorMessage, innerException)
+        {
+        }
+    }
+}

src/ImageProcessor/ParallelImageProcessor.cs

@@ -75,50 +75,58 @@ namespace ImageProcessor
         /// <inheritdoc/>
         public void Apply(ImageBase target, ImageBase source, int width, int height, Rectangle targetRectangle = default(Rectangle), Rectangle sourceRectangle = default(Rectangle))
         {
-            float[] pixels = new float[width * height * 4];
-            target.SetPixels(width, height, pixels);
-
-            if (sourceRectangle == Rectangle.Empty)
+            try
             {
-                sourceRectangle = source.Bounds;
-            }
-
-            // We don't want to affect the original source pixels so we make clone here.
-            ImageFrame frame = source as ImageFrame;
-            Image temp = frame != null ? new Image(frame) : new Image((Image)source);
-            this.OnApply(temp, target, target.Bounds, sourceRectangle);
+                float[] pixels = new float[width * height * 4];
+                target.SetPixels(width, height, pixels);
 
-            targetRectangle = target.Bounds;
-            this.numRowsProcessed = 0;
-            this.totalRows = targetRectangle.Bottom;
+                if (sourceRectangle == Rectangle.Empty)
+                {
+                    sourceRectangle = source.Bounds;
+                }
 
-            if (this.Parallelism > 1)
-            {
-                int partitionCount = this.Parallelism;
+                // We don't want to affect the original source pixels so we make clone here.
+                ImageFrame frame = source as ImageFrame;
+                Image temp = frame != null ? new Image(frame) : new Image((Image)source);
+                this.OnApply(temp, target, target.Bounds, sourceRectangle);
 
-                Task[] tasks = new Task[partitionCount];
+                targetRectangle = target.Bounds;
+                this.numRowsProcessed = 0;
+                this.totalRows = targetRectangle.Bottom;
 
-                for (int p = 0; p < partitionCount; p++)
+                if (this.Parallelism > 1)
                 {
-                    int current = p;
-                    tasks[p] = Task.Run(() =>
-                    {
-                        int batchSize = targetRectangle.Bottom / partitionCount;
-                        int yStart = current * batchSize;
-                        int yEnd = current == partitionCount - 1 ? targetRectangle.Bottom : yStart + batchSize;
+                    int partitionCount = this.Parallelism;
 
-                        this.Apply(target, temp, targetRectangle, sourceRectangle, yStart, yEnd);
-                    });
+                    Task[] tasks = new Task[partitionCount];
+
+                    for (int p = 0; p < partitionCount; p++)
+                    {
+                        int current = p;
+                        tasks[p] = Task.Run(() =>
+                        {
+                            int batchSize = targetRectangle.Bottom / partitionCount;
+                            int yStart = current * batchSize;
+                            int yEnd = current == partitionCount - 1 ? targetRectangle.Bottom : yStart + batchSize;
+
+                            this.Apply(target, temp, targetRectangle, sourceRectangle, yStart, yEnd);
+                        });
+                    }
+
+                    Task.WaitAll(tasks);
+                }
+                else
+                {
+                    this.Apply(target, temp, targetRectangle, sourceRectangle, targetRectangle.Y, targetRectangle.Bottom);
                 }
 
-                Task.WaitAll(tasks);
+                this.AfterApply(temp, target, target.Bounds, sourceRectangle);
             }
-            else
+            catch (Exception ex)
             {
-                this.Apply(target, temp, targetRectangle, sourceRectangle, targetRectangle.Y, targetRectangle.Bottom);
-            }
 
-            this.AfterApply(temp, target, target.Bounds, sourceRectangle);
+                throw new ImageProcessingException($"An error occured when processing the image using {this.GetType().Name}. See the inner exception for more detail.", ex);
+            }
         }
 
         /// <summary>