Add descriptive message for kit domain restrictions

[mlwilkerson]

When loading the icon chooser, if the configured kit has domain restrictions that exclude the current domain, the request for pro.min.js (and any other assets) will result in a 403. The icon chooser currently shows a vague failure error message.

But it could and should present a message suggesting that the user add a domain allowance for the current domain in their kit settings in the fontawesome.com account.

[mlwilkerson]

This will require changing how messaging works in fa-icon-chooser. Right now there's just a pair of slots for any fatal error: fatal-error-heading and fatal-error-detail. So there's no way from the client perspective to vary the message displayed based on the various possible results from the handleQuery or getUrlText callbacks.

To solve it might require updating their APIs to return errors with descriptive messages that can be displayed in the fa-icon-chooser UI.