Handle API errors by not exporting "outdated" metrics

[jamesread]

          Hey @lapo-luchini , gmail / google is kinda shitty with their APIs - things seem to randomly change and aren't versioned great. It does look like some scopes were renamed which I need to look into - see https://github.com/jamesread/prometheus-gmail-exporter/issues/32#issuecomment-2005173128 . 

I agree that this app should expose this as an error to prom - I don't know what is the idiomatic way to do this in Prometheus, so I'll start some research on that.

Originally posted by @jamesread in #31 (comment)