Unfriendly error thrown if you try to publish api backend changes in admin without checking any

[GUI]

• In the admin, modify an API backend so there are pending changes to publish.
• Head to the Publish Changes screen.
• Make sure none of the checkboxes are checked (no checkboxes will be checked by default if you have multiple pending changes, but if there's just a single pending change, it will be checked by default).
• Hit the big "Publish" button.
• You'll get this unfriendly error that doesn't really explain things:
  

This only happens if you haven't checked any checkboxes, so it's usually pretty easy to catch, but the error should explain what's going on and maybe prevent the publish button from even being active until you check something.

This scenario also triggers an uncaught exception on the backend publish API (Pundit::AuthorizationNotPerformedError) that would be good to cleanup.

[GUI]

This has been fixed (NREL/api-umbrella@f0b9315) and pushed live. The publish button is no longer clickable until you check at least one item. The underlying API also no longer errors out if no items are selected for publishing.