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.
Mmm, sorry what do you mean by "async" here? If you mean that these enum strings can be changed in Gecko (I guess it's rare case), then using enum at gaia side will only help (string should be updated only in one place) or what do you mean?