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 don't understand properly: all you're saying is fulfilled by using direct strings in errors.js (and I agree). I think errors.js is really good, what I don't like is the enum in MessageManager.ErrorCode and ThreadUI.ErrorCode.

> 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);

> For every enum member we can add comment about this error code (if required of course);

It's also the case with strings in errors.js

> In case error name is changed (probably rare case), you don't need to updated it everywhere, update is needed in on place.

For Gecko errors, using strings would be the same anyway. Except that for someone that doesn't know the code well, it's easier to check in errors.js than in MessageManager. (but that's arguable).