Make exception message even friendlier

Author: jcansdale

src/GitHub.App/Services/PullRequestService.cs

@@ -321,7 +321,7 @@ public IObservable<string> ExtractFile(
                     }
                     catch (NotFoundException ex)
                     {
-                        throw new NotFoundException($"Couldn't find merge base. Please check your network connection and try again.", ex);
+                        throw new NotFoundException($"The Pull Request file failed to load. Please check your network connection and click refresh to try again.", ex);
                     }
                 }
 
@@ -552,4 +552,4 @@ static Tuple<string, int> ParseGHfVSConfigKeyValue(string value)
             return null;
         }
     }
-}
+}