chrt: validate priority before trying to use it

Earlier message:

$ chrt -i 1 ls
chrt: failed to set pid 0's policy: Invalid argument

basically told 'something failed', while the new one tries to be more
helpful.

$ chrt -i 1 ls
chrt: unsupported priority value for the policy: 1: see --max for valid range

Addresses: https://bugs.debian.org/791707
Signed-off-by: Sami Kerola <kerolasa@iki.fi>

schedutils/chrt.c

@@ -504,7 +504,11 @@ int main(int argc, char **argv)
 #endif
 	if (ctl->pid == -1)
 		ctl->pid = 0;
-
+	if (ctl->priority < sched_get_priority_min(ctl->policy) ||
+	    sched_get_priority_max(ctl->policy) < ctl->priority)
+		errx(EXIT_FAILURE,
+		     _("unsupported priority value for the policy: %d: see --max for valid range"),
+		     ctl->priority);
 	set_sched(ctl);
 
 	if (ctl->verbose)