Display appropriate Sign-in Errors

[elauria]

When the sign-in process fails for any given reason (i.e. user doesn't have ZMON role),
he login process becomes an infinite loop on the 'Select your google account' page.

How to Reproduce:
1.- Logout from ZMON if already logged in.
2.- Open ZMON in browser
3.- A list of google accounts will be shown. Choose one that doesn't have a ZMON role.
4.- Authentication Fails but no message/screen is shown
5.- List of google accounts is shown again.

Whenever an error occured in the loggin process on ZMON side (not google), a message should be displayed informing the user. In the case of missing a role, would be helpful if it points it out.