Fixed never used warning.

src/ImageSharp/ImageProcessor.cs

@@ -39,11 +39,13 @@ namespace ImageSharp.Processing
 
                 this.AfterApply(source, sourceRectangle);
             }
-            catch (Exception ex)
-            {
 #if DEBUG
+            catch (Exception)
+            {
                 throw;
 #else
+            catch (Exception ex)
+            {
                 throw new ImageProcessingException($"An error occured when processing the image using {this.GetType().Name}. See the inner exception for more detail.", ex);
 #endif
             }