A failed identifier validation leaves the image style creation form unusable

Problem

The "New Image Style" field of the image style administration checks the entered name against the entitynamevalidation_json page, and only enables the creation button once that check has answered. When the request fails, or returns something unexpected, nothing ever re-enables the button. The request also has no timeout, so a validation page that never answers blocks the form indefinitely.

Steps to reproduce

• Go to Administration / Content / Image Styles
• Make the entity name validation request fail (for instance by cutting the network connection), or have it never answer
• Type a name in the "New Image Style" field

Actual result

The creation button stays disabled forever, with no way to recover other than reloading the page. Nothing tells the user that the name could not be checked - the failure is only visible in the browser console, and even there it is reported with console.log instead of console.error.

Expected result

The administration falls back to the name as entered, the creation button gets enabled, and the user is told that the name could not be checked and will be used as it was typed.