Service::MountFileSystemInternal calls chromeos::file_system_provider::Observer is inconsistently. Sometimes its called when there is an error, but sometimes it isn't.

We should fix the places where it isn't called, and call it with an error code.
 

chromeos::file_system_provider::Observer is called inconsistently. Sometimes its called when there is an error, but sometimes it isn't. For example, in Service::MountFileSystemInternal we always call the observer for each error, however in _api.cc we don't.

We should fix the places where it isn't called, and call it with an error code.