You probably can return the path directly without using a local variable :)

Retunring a non zero error code when problems are found seems like a good idea to me.

Yes, but this is the same problem encountered with the previous linter. If the script returns non zero, arcanist interprets as "the script did not run correctly" instead of "some linter errors were found".
Arcanist expects the linter to output to stdout and returns 0 if the output should be parsed; otherwise it throws an exception.

Unfortunately not, this is the default behavior of arcanist (I should remove these lines, they are redondant).
Even if you enforce :

if($err != 0) {
  $err = 0;
}

, arcanist will throw an exception anyway.
It actually makes sense if you think it as a linter which is designed to get its output parsed. How would arcanist know that it's parsing a correct output otherwise ?

It's probably better to keep the check then, to make sure you don't try to parse garbage.