fix(kconfig): surface generator errors from loadconfig

ensure_env() sent the child's stdout to DEVNULL to keep this process's stdout
parseable, but the generator reports missing imports with print(), so that was
exactly the output being discarded. What survived was a CalledProcessError
whose message repeats the argv, and the argv carries every .msg path: 9901
bytes on one line, naming no cause and no remedy.

Capture both streams and, on failure, exit with the child's own output. The
message names the command that failed and its exit code rather than asserting
what kind of failure it was, so it stays accurate whatever the child is or why
it failed. Missing empy now reports 167 bytes:

  loadconfig: px_generate_zenoh_topic_files.py failed (exit 1)
  Failed to import em: No module named 'em'

  You may need to install it using:
      pip3 install --user empy

stdout stays clean on success, so callers that parse it are unaffected.

Smoke tested on Ubuntu 24.04, matching the runner base: happy path returns
valid JSON for --group and --group --seeders; missing empy and missing
pyros-genmsg each report the cause and remedy; an unrunnable generator reports
the interpreter's own "can't open file"; a child exiting non-zero with no
output reports "(no output)" rather than an empty message; and a preset
ZENOH_KCONFIG_TOPICS still skips the generator entirely.

Assisted-by: Claude:claude-opus-5
Signed-off-by: Ramon Roche <mrpollo@gmail.com>

Tools/kconfig/loadconfig.py

@@ -46,10 +46,18 @@ def ensure_env():
     atexit.register(shutil.rmtree, out_dir, ignore_errors=True)
     msg_files = sorted(glob.glob(os.path.join(PX4_ROOT, 'msg', '*.msg')) +
                        glob.glob(os.path.join(PX4_ROOT, 'msg', 'versioned', '*.msg')))
-    # stdout must stay clean: several callers print machine-parsed output
-    subprocess.run([sys.executable, _ZENOH_GENERATOR, '--zenoh-config',
-                    '-f'] + msg_files + ['-o', out_dir, '-e', _ZENOH_TEMPLATES],
-                   check=True, stdout=subprocess.DEVNULL)
+    # stdout must stay clean: several callers print machine-parsed output.
+    # Capture rather than discard it, because the generator reports missing
+    # imports on stdout; discarding left a CalledProcessError whose argv dump
+    # (every .msg path, ~9kB on one line) buried the actual cause.
+    result = subprocess.run([sys.executable, _ZENOH_GENERATOR, '--zenoh-config',
+                             '-f'] + msg_files + ['-o', out_dir, '-e', _ZENOH_TEMPLATES],
+                            stdout=subprocess.PIPE, stderr=subprocess.PIPE, text=True)
+    if result.returncode != 0:
+        detail = ((result.stdout or '') + (result.stderr or '')).strip()
+        sys.exit('loadconfig: {} failed (exit {})\n{}'.format(
+            os.path.basename(_ZENOH_GENERATOR), result.returncode,
+            detail or '(no output)'))
     os.environ['ZENOH_KCONFIG_TOPICS'] = os.path.join(out_dir, 'Kconfig.topics')