Omniprobe wrapper silently masks child-process exit codes

[lwitschen]

Summary

omniprobe/omniprobe::capture_subprocess_output reads the wrapped
child's stdout/stderr but does not propagate the child's exit code.
Combined with the top-level invocation in the same file, which discards
any return value, every wrapped command exits 0 regardless of how the
child terminated (non-zero exit, SIGABRT, SIGSEGV, etc.).

Evidence

• omniprobe/omniprobe:115-184 (capture_subprocess_output): the
  comment # Get process return code on line 165 is followed by
  buffer-draining logic only; process.returncode is never read or
  returned.
• omniprobe/omniprobe:788: capture_subprocess_output(parms.remaining[1:], ...)
  discards any return value; no sys.exit() with the child's status.

Expected behavior

Wrapper exit code mirrors the child's. Signal terminations surface as
128 + signo (or whatever convention the harness chooses).

Impact

• CI cannot rely on the wrapper exit code to detect instrumented-child
  failures; consumers have to grep stderr for diagnostic strings.
• Downstream test scripts that gate on omniprobe exit status silently
  pass on instrumentation crashes.

Suggested fix

1. capture_subprocess_output: return process.returncode (and a
   sensible value for the KeyboardInterrupt path, e.g. 130).
2. Top-level invocation: sys.exit(capture_subprocess_output(...)).