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.
Ok, let me describe all benefits I see using "enum" approach and if you don't agree or benefits are insignificant for you, I'll proceed with strings then :) :

* All related strings are located in one place, where you can see what possible errors are expected; They are logically grouped. It's easy to find how particular error is handled (I don't need to guess what error code string could be for the failed FDN check for example);
* Namespace, your proposition has the same benefit;
* For every enum member we can add comment about this error code (if required of course);
* In case error name is changed (probably rare case), you don't need to updated it everywhere, update is needed in on place.

To be honest, the first two seem significant for me, the rest aren't.

Also I'm not sure how do you propose to add name space prefixes for the errors coming from Messaging API? What I have in mind doesn't look like a small and easy change :)