chrt: produce better error message for missing priority with implied -r

chrt: produce better error message for missing priority with implied -r

Since commit 4c425142844d, the following two equivalent commands produce
different error messages:

  # chrt -r ls NEWS
  chrt: invalid priority argument: 'ls'
  # chrt ls NEWS
  chrt: unsupported priority value for the policy: 0: see --max for valid range

The latter error message is enigmatic: where did the user specify '0'?

Before the mentioned commit, the second command would produce the same
error message as the first.  Restore that behavior.

CC: Madadi Vineeth Reddy <vineethr@linux.ibm.com>
Signed-off-by: Benno Schulenberg <bensberg@telfort.nl>