Tweak incorrect API scopes exception message.

The previous message wasn't very useful as it would only be displayed when a user was logging in with a token, in which case signing out and back in _wouldn't_ fix the problem.

Author: Steven Kirk

src/GitHub.Api/IncorrectScopesException.cs

@@ -4,13 +4,13 @@
 namespace GitHub.Api
 {
     /// <summary>
-    /// Thrown when the login for a user does not have the required scopes.
+    /// Thrown when the login for a user does not have the required API scopes.
     /// </summary>
     [Serializable]
     public class IncorrectScopesException : Exception
     {
         public IncorrectScopesException()
-            : this("You need to sign out and back in.")
+            : this("Incorrect API scopes.")
         {
         }
 

src/GitHub.Api/LoginManager.cs

@@ -356,7 +356,8 @@ async Task<User> GetUserAndCheckScopes(IGitHubClient client)
                 log.Error("Error reading scopes: /user succeeded but scopes header was not present");
             }
 
-            throw new IncorrectScopesException();
+            throw new IncorrectScopesException(
+                "Incorrect API scopes. Required: " + string.Join(",", scopes));
         }
 
         Uri GetLoginUrl(IOauthClient client, string state)