goaoauthprovider: fix expected NULL GError
If the requested session keyring alias doesn't exist, the call to retrieve it returns NULL without setting the GError.
Detect this case and report the failure to the user.
cc #376
Edited by Andy Holmes
If the requested session keyring alias doesn't exist, the call to retrieve it returns NULL without setting the GError.
Detect this case and report the failure to the user.
cc #376