Provide response status code in case of error

[aleksey-korolev]

Is your feature request related to a problem? Please describe.
We'd like to be able to separately monitor different error types such as quota exceeded which isn't possible since status code is not returned by the go client.
See https://github.com/googlemaps/google-maps-services-go/blob/master/client.go#L384

Describe the solution you'd like
It would be nice to be able to get status code of error, not just error string, in high-level functions such as Geocode(): https://github.com/googlemaps/google-maps-services-go/blob/master/geocoding.go#L35

Describe alternatives you've considered
Alternative we are using now is searching of status code substring such as OVER_QUERY_LIMIT in returned error string.