POSIX says grep exit code should be 2 (or greater) when an error occurs.

http://pubs.opengroup.org/onlinepubs/9699919799/utilities/grep.html

   * 0 One or more lines were selected.
   * 1 No lines were selected.
   * >1 An error occurred.

That's how GNU grep behaves:

    $ grep '\(' /etc/passwd; echo $?
    grep: Unmatched ( or \(
    2

But Busybox is currently returning 1, which is the exit code we rely to check if the pattern not matched:

    / # grep '\(' /etc/passwd; echo $?
    grep: bad regex '\(': Missing ')'
    1

Could you please make it return 2 in case of errors?