Improve error handling around job retry with conflict

[brandur]

Fixes #622, which indicates how when retrying a job and getting a unique
conflict, River UI produces an internal server error instead of a proper
one that's raised to the user.

[brandur]

@bgentry Not going to lie, most of this is Codex. I had Codex do another review pass locally and it seems to think the changes are okay.

[bgentry]

Hah, it threw a nice refactor in there too 🤩 There is a ton of low hanging fruit in riverui that modern AIs can absolutely plow through, don't be afraid to go to town shipping anything on it. It's a simple, conventional React app with good foundations and not too much special stuff going on other than the pro/OSS split (but that's really just basic feature detection as far as frontend is concerned).

[brandur]

There is a ton of low hanging fruit in riverui that modern AIs can absolutely plow through, don't be afraid to go to town shipping anything on it. It's a simple, conventional React app with good foundations and not too much special stuff going on other than the pro/OSS split (but that's really just basic feature detection as far as frontend is concerned).

Oh yesssss, +1. Totally true — AI could probably make some vast improvements. I'll write that down to try at some point.