[Code review changes 2] (Added new token-related handling to call erl_services callback, invoked in case of bad token or token update)
1. No error/warning when feedback callback is missing, since it's a 3rd party and up to consumer to decide whether to implement feedback callback or not.
2. Changed the handle_error guard to use proper syntax ';' instead of 'orelse'.
3. Aligned the comment style to be consistent.
4. gcm:handle_error_generic/3 -- gcm:handle_error_generic/2, note the 'undefined' to '_' change at the end, to make sure it doesn't crash when host process uses a wrong type for the feedback env setting.
5. gcm_request:parse_results/3 -- Added recursive call in last case pattern, to avoid ignoring other results when an unexpected response found.