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.
I mean, it's difficult to keep this synchronized with the real code, or at least, easier to forget to update both places