Update FxA client and manager with latest gecko changes
Use FxAccountsClient
Hook up to the FxA client!

* Start to hook up error screens based on responses.
* Rename FxModuleServerRequest->checkPassword to signIn, and createAccount->signUp
Start to generalize the error handling.

* Create fxa_errors.js which converts error responses to strings
* Add showResponse in fxa_error_overlay.js which shows error responses.
* Start to unify how all errors are handled.