1.4.1 Updates and improvements to error checking

Made changes based on PR https://github.com/zendesklabs/ticket_field_manager_app/pull/9
I've also added in a conditional on AJAX fail errors to stop error messages being displayed if the call is aborted.

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4.1.1 - Error handling and better support for multiple Ticket Field Manager apps in one account

Updated app to include better and more useful error handling.

The app will also use the metadata store across apps to cache REST API calls to the /users/{id}.json end-point (for current user). This will help reduce save hook timeouts. The app will however make calls to the /users/{id}.json on App.activated to ensure data is up-to-date
What happens if I call 4 times `this.currentUserData()` and `this.store('ticketfieldmanager_user')` is empty? Looks like it will make 4 api call to `fetchUser`. Don't you think it'll make sense to flag that we're already making the request so we don't make unnecessary api requests?