Fix settings rendering for `extra-build-dependencies` (#15622)
## Summary
This was fixed in https://github.com/astral-sh/uv/pull/15161, then
reverted as it regressed the error handling. I've re-applied the change
here, but moved the error handling to the runtime, rather than
parse-time. I think this is slightly worse in that we no longer include
the originating source code snippet, but it at least gives us the
expected behavior :(
Closes https://github.com/astral-sh/uv/issues/15124.
