Improve logging of command execution failures

Previously the mailman command output was sent to /dev/null.

mailman_api/mailman.py

@@ -32,7 +32,16 @@ mm_env['LANG'] = 'C'
 mm_env['LC_ALL'] = 'C'
 
 def _run(cmd):
-    subprocess.check_call(cmd, env=mm_env)
+    app.logger.info('running %s', cmd)
+    try:
+        subprocess.run(
+            cmd, check=True, env=mm_env,
+            stdout=subprocess.PIPE,
+            stderr=subprocess.STDOUT)
+    except subprocess.CalledProcessError as e:
+        app.logger.error('command FAILED (status=%d), output:\n%s\n',
+                         e.returncode, e.stdout.decode('utf-8'))
+        raise
 
 
 def _random_password():