Add warnings when blob URL file can't be found

Warn when blob can't be found in the repository.
Warn when the working file is different blob URL file.

Author: jcansdale

src/GitHub.App/Services/GitHubContextService.cs

@@ -3,6 +3,7 @@
 using System.Text;
 using System.Linq;
 using System.Windows;
+using System.Globalization;
 using System.Threading.Tasks;
 using System.Collections.Generic;
 using System.ComponentModel.Composition;
@@ -75,13 +76,19 @@ public void TryNavigateToContext(string repositoryDir, GitHubContext context)
         {
             if (context?.LinkType == LinkType.Blob)
             {
-                // Navigate to file for /blob/ type URLs
                 var (commitish, path, commitSha) = ResolveBlob(repositoryDir, context);
+                if (commitish == null && path == null)
+                {
+                    var message = string.Format(CultureInfo.CurrentCulture, Resources.CouldntFindCorrespondingFile, context.Url);
+                    vsServices.ShowMessageBoxInfo(message);
+                    return;
+                }
 
                 var hasChanges = HasChangesInWorkingDirectory(repositoryDir, commitish, path);
                 if (hasChanges)
                 {
-                    vsServices.ShowMessageBoxInfo(Resources.ChangesInWorkingDirectoryMessage + Environment.NewLine + commitish);
+                    var message = string.Format(CultureInfo.CurrentCulture, Resources.ChangesInWorkingDirectoryMessage, commitish);
+                    vsServices.ShowMessageBoxInfo(message);
                 }
 
                 TryOpenFile(repositoryDir, context);

src/GitHub.Resources/Resources.Designer.cs

@@ -810,7 +810,7 @@ https://git-scm.com/download/win</value>
     <value>There is no active repository to navigate</value>
   </data>
   <data name="ChangesInWorkingDirectoryMessage" xml:space="preserve">
-    <value>This file has changed since the permalink was created</value>
+    <value>The working file is different to the file at '{0}'. Please checkout the corresponding branch, pull request or commit.</value>
   </data>
   <data name="DifferentRepositoryMessage" xml:space="preserve">
     <value>Please open the repository '{0}' and try again</value>
@@ -845,4 +845,7 @@ https://git-scm.com/download/win</value>
   <data name="RepositoriesMustHaveRemoteOriginHowToFix" xml:space="preserve">
     <value>Please rename one of your existing remotes to 'origin' or add a new remote named 'origin' and fetch. This can be done from the command line or by clicking the button below.</value>
   </data>
+  <data name="CouldntFindCorrespondingFile" xml:space="preserve">
+    <value>Couldn't find file corresponding to '{0}' in the repository. Please do a 'git fetch' or checkout the target pull request.</value>
+  </data>
 </root>