Proper error message for 'git gui' on Mac

GVFS/GVFS.Hooks/HooksPlatform/GVFSHooksPlatform.Mac.cs

@@ -23,5 +23,10 @@ public static string GetNamedPipeName(string enlistmentRoot)
         {
             return MacPlatform.GetNamedPipeNameImplementation(enlistmentRoot);
         }
+
+        public static string GetGitGuiBlockedMessage()
+        {
+            return "git gui is not supported in VFS for Git repos on Mac";
+        }
     }
 }

GVFS/GVFS.Hooks/HooksPlatform/GVFSHooksPlatform.Windows.cs

@@ -46,5 +46,10 @@ public static string GetUpgradeHighestAvailableVersionDirectory()
         {
             return WindowsPlatform.GetUpgradeHighestAvailableVersionDirectoryImplementation();
         }
+
+        public static string GetGitGuiBlockedMessage()
+        {
+            return "To access the 'git gui' in a GVFS repo, please invoke 'git-gui.exe' instead.";
+        }
     }
 }

GVFS/GVFS.Hooks/Program.cs

@@ -132,7 +132,7 @@ private static void CheckForLegalCommands(string[] args)
             switch (command)
             {
                 case "gui":
-                    ExitWithError("To access the 'git gui' in a GVFS repo, please invoke 'git-gui.exe' instead.");
+                    ExitWithError(GVFSHooksPlatform.GetGitGuiBlockedMessage());
                     break;
             }
         }