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 believe we'll have more Thread specific errors soon (at least I think we'll need one for https://bugzilla.mozilla.org/show_bug.cgi?id=990021)