Extend run-tests.ps1 by a warning if submodules are not synced

I'm contributing to ImageSharp since around a week, and I saw 3 people (myself included) stumbling over failing tests due to the submodule for test data being out of sync.

With this commit the run-tests.ps1 test script is extended by the following behaviour:
- if something fails during the script run, the submodule status is checked.
  A warning is printed if
  - any submodule is not initialized or
  - any submodule is not synchronized
  - git could not be called to check the submodule status.

run-tests.ps1

@@ -15,6 +15,30 @@ function VerifyPath($path, $errorMessage) {
     }
 }
 
+function CheckSubmoduleStatus() {
+  $submoduleStatus = (git submodule status) | Out-String
+  # if the result string is empty, the command failed to run (we didn't capture the error stream)
+  if ($submoduleStatus) {
+    # git has been called successfully, what about the status?
+    if ($submoduleStatus -contains '-')
+    {
+	  # submodule has not been initialized!
+	  return 2;
+    } elseif ($submoduleStatus -contains '+')
+    {
+      # submodule is not synced:
+	  return 1;
+    } else {
+	  # everything fine:
+	  return 0;
+	}
+  } else {
+    # git call failed, so we should warn
+	return 3;
+  }
+}
+
+
 if ( ($targetFramework -eq "netcoreapp2.0") -and ($env:CI -eq "True") -and ($is32Bit -ne "True")) {
     # We execute CodeCoverage.cmd only for one specific job on CI (netcoreapp2.0 + 64bit )
     $testRunnerCmd = ".\tests\CodeCoverage\CodeCoverage.cmd"
@@ -64,4 +88,22 @@ Invoke-Expression $testRunnerCmd
 
 cd $PSScriptRoot
 
+
+if (0 -ne ([int]$LASTEXITCODE)) {
+  # check submodule status
+  $submoduleStatus = CheckSubmoduleStatus
+  if ([int]$submoduleStatus -eq 1) {
+    # not synced
+	Write-Host -ForegroundColor Yellow "Check if submodules are up to date. You can use 'git submodule update' to fix this";
+  } elseif ($submoduleStatus -eq 2) {
+    # not initialized
+	Write-Host -ForegroundColor Yellow "Check if submodules are initialized. You can run 'git submodule init' to initialize them."
+  } elseif ($submoduleStatus -eq 3) {
+    # git not found, maybe submodules not synced?
+	Write-Host -ForegroundColor Yellow "Could not check if submodules are initialized correctly. Maybe git is not installed?"
+  } else {
+    #Write-Host "Submodules are up to date";
+  }
+}
+
 exit $LASTEXITCODE