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.
wondering how much it's better than `return ERRORS.get(errorCode) || ERRORS.get(MessageManager.ErrorCode.UNKNOWN_ERROR);` or if it's completely equivalent.

(I'm not asking to change it, both are fine if they work IMO)