Information about authenticate_user_login() failure

**Problem**
When the user is registered by any other authentication plugin (manual, email ...) and try to login via OAuth2, then it is silently redirected to the login page, without knowing what happened.

**How to simulate**
Create a  user (manual or email-based), with a Gmail account.
Then try to login via OAuth2 plugin with the same email. (like using Google or Facebook account with same email)

**What's happenning**
The user will be silently redirected to login page.

**What's needed**
It is necessary to catch this condition and show correct information to the user.

**How this path works**
This patch displays instructions about the error if the user can not authenticate via `authenticate_user_login()` call. In this case possible cause is the e-mail address registered by another authenticate plugin.
Also, will be shown links to login and password recovery pages.