Improve error handling when uploading emojis due to configuration

Summary

When looking to add a custom emoji via the site administration (I.E. https://site.atlassian.net/admin) > Emojis, if the Allow people to add custom emojis toggle is off, although the user will be able to select the file, give it a name and tap the Upload button, a warning with an upload failed message will be displayed:

From the browser's console, a "403" is triggered and also an error message:

Request URL: https://admin.atlassian.com/gateway/api/emoji/CLOUD-ID/site/token/upload
Request Method: GET
Status Code: 403 
Remote Address: XXXXXXXXX
Referrer Policy: strict-origin-when-cross-origin

Unable to upload emoji: No media api support is configured

Suggestion

Since the error message is not clear, this is a suggestion to provide a message stating that the option to allow custom emojis is toggled off, needing to be toggled on.

This way, it quickly allows the administrators to understand what is the nature of the issue. Alternatively, not allowing the upload to happen until the option is toggled on would help as well.