Improve error message on missing permission

Improve error message on missing permission

In case "cmd" (and "args") in doas.conf(5) mismatch, the log syslog(3)
message might be read as if the command was executed but failed, i.e.
returned non-zero.

Be unambiguous and help admins spot execution *attempts* as such:

-Oct  9 01:05:20 eru doas: failed command for kn: echo bar
+Oct  9 01:05:20 eru doas: command not permitted for kn: echo bar

OK tedu deraadt