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.
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