fix: fail on unknown x.py commands

[Ehtasham-Yasin]

Summary

Make x.py return a failure status for unsupported commands while preserving its existing diagnostic output.

Motivation

Fixes #823. Invalid commands currently print an error but exit successfully, which makes scripts treat failed invocations as successful.

Target and compatibility impact

Only invalid CLI command dispatch changes: unsupported commands now exit with status 1. Supported commands, help/usage output, and target selection are unchanged.

Validation

• python3 -m unittest tools.test_x
• python3 -m py_compile x.py tools/test_x.py
• git diff --check

Checklist

• Commit includes a DCO Signed-off-by line.
• Test covers the new behavior.
• User-facing diagnostics are preserved; no documentation update is needed.
• The change does not affect the compiler/std/ license boundary.

[Ehtasham-Yasin]

CI investigation: the three failing WebAssembly jobs reproduce on the current master commit b384e64 in run 36709793033, before this PR. The failures report WebAssembly.instantiate(): Import #0 module="env" function="printf": function import requires a callable; this PR only changes the unsupported-command branch in x.py and cannot affect that runtime path.

[Ehtasham-Yasin]

Additional CI investigation: the Windows amd64/arm64 failures are existing test portability issues, not caused by this PR. Both jobs fail assertions that expect POSIX paths such as examples/std/example.wave and missing/wavec, while Windows correctly reports backslash-separated paths. The Windows packaging steps complete successfully. The WebAssembly failures remain the pre-existing env.printf runtime-import issue documented above.