Display error message when Media Library upload fails

[aduth]

Currently, when an upload fails in the media library, the following message is displayed:

An error occurred in the upload. Please try again later.

Behind the scenes, more useful error messages are set into the failure response, but they're not being exposed to the user. For example, /wp-admin/includes/ms.php returns error messages related to unavailable disk space, but only the message above is displayed.

Attached is a patch to display the error response to the user alongside the generic error message above.

[aduth]

Patch to display error response on a failed upload

[gcorne]

I think that a cleaner solution (see 29891-01.patch​} to this issue is to bypass calling wp_die() if during an ajax request. By not calling wp_die() the errors will flow through to wp_ajax_upload_attachment where they will result in an error JSON response.

[gcorne]

Related to #25849

[wonderboymusic]

In 30156:

Display error message when Media Library upload fails.

Props gcorne.
Fixes #29891.

[TobiasBg]

Let's use ! DOING_AJAX as we do everywhere else, see 29891.2.diff​.

[jeremyfelt]

In 30177:

Use common core syntax for checking DOING_AJAX

Our current check for a non truthy DOING_AJAX is ! DOING_AJAX.

props TobiasBg
Fixes #29891