better UX for Anthropic (or insert upstream $model_provider) API request errors

[banksean]

I see this "..." chat bubble in the messages view, and it's just been sitting there like this for quite a while:

When I check the logs, I can see that there was a 500 response from Anthropic with the message, "anthropic request sleep before retry"

{"time":"2025-07-17T20:59:51.708287224Z","level":"WARN","msg":"anthropic_request_failed","response":"{\"type\":\"error\",\"error\":{\"type\":\"api_error\",\"message\":\"Internal server error\"}}","status_code":500,"session_id":"dg44-4nh5-tpnd-n1bn","convo_id":"304-pbzc"}
{"time":"2025-07-17T20:59:51.70839789Z","level":"WARN","msg":"anthropic request sleep before retry","sleep":30424530093,"attempts":1,"session_id":"dg44-4nh5-tpnd-n1bn","convo_id":"304-pbzc"}
{"time":"2025-07-17T21:00:32.613056177Z","level":"WARN","msg":"anthropic_request_failed","response":"{\"type\":\"error\",\"error\":{\"type\":\"api_error\",\"message\":\"Internal server error\"}}","status_code":500,"session_id":"dg44-4nh5-tpnd-n1bn","convo_id":"304-pbzc"}
{"time":"2025-07-17T21:00:32.613164469Z","level":"WARN","msg":"anthropic request sleep before retry","sleep":60848943001,"attempts":2,"session_id":"dg44-4nh5-tpnd-n1bn","convo_id":"304-pbzc"}

We should at least let the user know that Sketch got their message, but Sketch is having problems talking to Anthropic. Sketch will keep trying for some time, but the issue isn't due to Sketch itself.