Fix unhandled exception if code coverage results file does not exist.

Show error message in code coverage pad's text editor if the code coverage file does not exist.

src/AddIns/Analysis/CodeCoverage/Project/Src/CodeCoverageControl.cs

@@ -318,7 +318,9 @@ void ListViewItemActivate(object sender, EventArgs e)
 		void OpenFile(string fileName, int line, int column)
 		{
 			if (fileName != textEditorFileName) {
-				textEditor.Load(fileName);
+				if (!TryLoadFileIntoTextEditor(fileName)) {
+					return;
+				}
 				textEditor.SyntaxHighlighting = GetSyntaxHighlighting(fileName);
 			}
 			textEditor.ScrollToEnd();
@@ -327,6 +329,17 @@ void OpenFile(string fileName, int line, int column)
 			CodeCoverageService.ShowCodeCoverage(new AvalonEditTextEditorAdapter(textEditor), fileName);
 		}
 
+		bool TryLoadFileIntoTextEditor(string fileName)
+		{
+			if (!File.Exists(fileName)) {
+				textEditor.Text = String.Format("File does not exist '{0}'.", fileName);
+				return false;
+			}
+
+			textEditor.Load(fileName);
+			return true;
+		}
+
 		IHighlightingDefinition GetSyntaxHighlighting(string fileName)
 		{
 			return HighlightingManager.Instance.GetDefinitionByExtension(Path.GetExtension(fileName));