[buildcore] Add error message to pybb debug for missing debugger

deps/buildcore/scripts/pybb.py

@@ -98,6 +98,9 @@ def debug(paths: List[str]) -> int:
         args = ['gdb', '--args']
     elif shutil.which('lldb') is not None:
         args = ['lldb', '--']
+    else:
+        sys.stderr.write('debug: could not find a supported debugger\n')
+        return 1
     args.extend(paths)
     return subprocess.run(args).returncode