Send user back to the login screen if credentials aren't working closes #1844

Pretty basic here if you want to review @oxtoacart. The theory with the prior change was that CredentialException was somehow being thrown when we in fact just couldn't find a proxy. That appears not to be the case, however, and we get CredentialException when the credentials are in fact incorrect. 

Ideally we'd add more messaging, but this will solve the problem I think fine, particularly given that the user has to actively invalidate her token and will likely expect something to not work as normal.