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.
not sure either, their value is quite small, yeah.

I'd be even more inclined to simple check that we can get the default value, and all of the others without checking their actual value, simply checking that it's not the default value.