Add error handling for the media manager Ajax response

[johnbillion]

When the media manager modal performs an Ajax request to query attachments, if the Ajax call responds with an error then the user is presented with an eternal loading spinner and no indication there has been a problem.

I think there are two places which need a fail() handler for the Ajax promise:

• wp.media.model.Attachments.more()
• wp.media.model.Query.more()

I haven't done any testing yet though to confirm this.

Needs a decision around how best to present an error to a user. Probably needs UI work; maybe one of the UI pieces such as the upload failure message can be reused.

[joemcgill]

Thanks for the report @johnbillion, let's add this to 5.5 to confirm and get some design feedback on the best user experience for this.