Passthrough error from custom generateToken for debugging
Add support for non-expiring tokens

New tokens will be considered to never expire if respective
accessTokenLifetime or refreshTokenLifetime are null.
Equally, if saveAccessToken/saveRefreshToken() methods return an
expires value of null, the token will be considered valid.
If a non-expiring token is being issued, the "expires_in" key will
be omitted from the response
v1.5.0