Bug 1035279 - [Messages][Refactoring] Move error codes mapping from dialog.js to a separate file. r=julien

* ErrorDialog moved to a separate file;
* Removed ErrorDialog dependency on Errors mapping;
* Errors mapping moved to a separate file;
* ErrorCodes organized into enums.
Agree that plain strings would be OK if we were using error codes only inside error.js, but we're using it in both error.js (to configure error message dialog) and thread_ui.js (to apply appropriate logic). If it needed for error.js only, then enum is definitely overkill.