hrw4u: no non-zero exit status on errors unless --stop-on-error is given

[zwoop]

While reviewing #13204, we realized that hrw4u generally doesn't exit with a status code on errors, unless --stop-on-error is given. This applies to u4wrh as well.

In the default error-collecting mode, every input error — syntax and semantic — is reported on stderr, partial output still goes to stdout, and the process exits 0:

$ uv run ./scripts/hrw4u /tmp/foo.hrw4u
cond %{TXN_CLOSE_HOOK} [AND]
/tmp/foo.hrw4u:2:4: error: Unknown statement function: 'counters'
   2 |     counters("my.stat");
     |     ^
$ echo $?
0

generate_output() gates sys.exit(1) on tree is None (tools/hrw4u/src/common.py:281), which with DefaultErrorStrategy never happens — the parser recovers and returns a tree. Only --stop-on-error (bail strategy) and I/O failures exit non-zero.

So nothing can wrap either tool: CI steps, Makefiles, and u4wrh in.hrw > out.hrw4u && hrw4u out.hrw4u all see success on a failed compile. In bulk in:out mode a broken artifact is left on disk under a name that looks converted.

Suggested fix, keeping collect-all behavior (the flag should control when we stop, not whether the run failed): return a failure flag from generate_output(), OR it across all inputs at the three call sites (common.py:366, :401, :429), and sys.exit(1) at the end of run_main(). Also skip the print at common.py:265 — or in in:out mode, don't write the output file at all — when errors were collected.