Show friendly error message if Git.exe not on PATH

VS 2017 will guide user to install Git for Windows as soon as Team Explorer is opened.
The user not having Git.exe on their PATH isn't the common case, so don't want to overengineer a solution.

Author: jcansdale

src/GitHub.App/Services/PullRequestService.cs

@@ -198,15 +198,28 @@ public IObservable<Unit> SyncSubmodules(ILocalRepositoryModel repository)
                 var exitCode = await SyncSubmodules(repository.LocalPath, line => output.WriteLine(line));
                 if (exitCode != 0)
                 {
-                    var ex = new ApplicationException(output.ToString());
+                    // Replace with friendly message if Git.exe isn't on path.
+                    // If culture isn't English, user will see the local equivalent of:
+                    // "'git' is not recognized as an internal or external command"                    
+                    var message = output.ToString();
+                    if (exitCode == 1 && message.StartsWith("'git' is not recognized as an internal or external command,", StringComparison.Ordinal))
+                    {
+                        message =
+@"Couldn't find Git.exe on PATH.
+
+Please install Git for Windows from:
+https://git-scm.com/download/win";
+                    }
+
+                    var ex = new ApplicationException(message);
                     return Observable.Throw<Unit>(ex);
                 }
 
                 return Observable.Return(Unit.Default);
             });
         }
 
-        // HACK: This is just a prototype!
+        // LibGit2Sharp has limited submodule support so shelling out Git.exe for submodule commands.
         async Task<int> SyncSubmodules(string workingDir, Action<string> progress = null)
         {
             var cmdArguments = "/C git submodule init && git submodule sync --recursive && git submodule update --recursive";