git-reviewrequest should check to ensure the token is valid

[f1sherman]

While requesting a review (git reviewrequest) I was receiving a 404 Resource Not Found error. After clearing out my token I was able to re-enter my password and the reviewrequest was successful. I assume that the cause of the initial error was that my token was no longer valid. We should check the validity of the stored token and request the users credentials if it is invalid.