Improve the user experience when Google credentials are expired or revoked

[parthea]

I think the user experience could be improved when Google credentials are expired or revoked. I see that we have a reauth parameter in read_gbq and to_gbq but this will cause the authentication to run each time even if the credentials are valid to cater for the environment where there are multiple users. It may be helpful to also have the ability to automatically trigger authentication flow when credentials are expired or revoked.

https://github.com/pydata/pandas-gbq/blob/master/pandas_gbq/gbq.py#L771

[tswast]

This is partially addressed by #212. It's now possible to refresh the credentials if the credentials are expired or revoked by manually setting reauth=True. Before that change, pandas-gbq would still try to load the expired credentials even when reauth=True and would fail with this error.

[parthea]

Closing as obsolete.