Logging in w/ google login fails if user has already created an account with the same gmail account

[jonpmarino]

Observed during RFT testing.

To reproduce:

1. I created a new account on mapstory.org using the normal login method. Username is johnglenn. I used a gmail account, glenn@mapstory.org, as the email account (this is a gmail account).

2. I confirmed and activated the account.

3. I logged off, cleared cookies and opened an incognito browser session

4. I went back to MapStory login and attempted to login with the Google social login.

5. I used the same gmail account previously used in normal login method, glenn@mapstory.org

6. I received this error message:

[emilyashley]

Would this tie in with #755 ?