fix(release-search): report a ValueError from a source as that source's error

[splitsec2]

Kept small on purpose so it's easy to review. This is the same kind of small fix as the ones in #1429 to #1432, so see the question in #1429 about separate PRs versus combined. I'm sending them separately until I hear otherwise.

In search_source_releases the except ValueError comes before the branch that handles the operational errors, and ValueError is in that tuple. A ValueError from build_release_search_plan or source.search() is therefore reported as Unknown source: <name> instead of <name>: <message>. The only ValueError meant for that branch is the one get_source raises for a name that isn't registered, so that call now has its own try and the rest goes back to the operational errors branch.

The failure test already covered OSError and RuntimeError, so I added ValueError to it, and that case fails on current main. Ruff, basedpyright and tests/core pass.

The one red check is the existing clock-dependent test test_a_queued_torrent_asks_for_a_grace_once, which fails on any runner that booted recently. It is fixed in #1426 and is not caused by this change.