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.
To get rid of jshint complaints on ```var dialog = new ErrorDialog(errorDescription);``` when ```dialog``` is not used. Do you have any better idea?