Keep error domain com.google.GIDSignIn across the whole library.

[brnnmrls]

At the moment, we have two error domains in the GSI (com.google.GIDSignIn and com.google.GIDVerify). Keeping the existing domain (com.google.GIDSignIn) will be the domain for all errors from this library while specific error codes will define the error. Though, the issue is avoiding collision across errors since error codes are solely integer backed enum cases and there's a chance integers are backed by the same one. We will revisit this to determine whether we should have two error domains or simplify to one.

Comment issue was brought up: https://github.com/google/GoogleSignIn-iOS/pull/414/files#r1599105257