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.
note: there is a separate bug to change this to a real list instead : https://bugzilla.mozilla.org/show_bug.cgi?id=944644