Handle additional errors from verifyIdToken #174

Is your feature request related to a problem? Please describe.

Firebase admin's verifyIdToken may throw some errors that this module should probably handle. See this thread for a background discussion: #125

See a list of Firebase admin auth errors here:
https://firebase.google.com/docs/reference/node/firebase.auth.Error

Currently, we only handle auth/id-token-expired.

Describe the solution you'd like and how you'd implement it

When calling verifyIdToken, this module should probably handle the following errors:

• auth/invalid-user-token: return an unauthed user
• auth/user-token-expired: return an unauthed user
• auth/user-disabled: return an unauthed user
• auth/argument-error: call to refresh the ID token—however, we need to confirm this will address the kid claim error (see this comment for more info).

Is this a breaking change?
Yes, in the case developers are catching and handling these errors themselves.

Describe alternatives you've considered
Continue to ignore errors and let developers handle them. I can't think of a compelling reason not to handle any of the above errors.