TypedResourceObjectAutocompleteWidget should distinguish between field being empty and remote source not available

Problem/Motivation

There is no distinction in the TypedResourceObjectAutocompleteWidget between the field being empty and the remote resource object being unavailable.

Both display an empty edit box. If the field does have a value it will be lost upon save. If the remote resource object is only temporarily unavailable, this will cause data loss.

Proposed resolution

Visually distinguish between these two states (details TBD).

Handle the situation where the field is saved when the remote resource object is unavailable - should this save the original value, or just warn the user that saving in the current state will clear the value of the field?

Remaining tasks

All the things.

User interface changes

TBD

API changes

None

Data model changes

None

Release notes snippet

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