ionice: non-cryptic error message when ionice can't execvp

Previously ionice would reply like this when it couldn't find the
command to execute:

    $ ionice -c 3 does-not-exist
    ionice: execvp failed: No such file or directory

Getting that message from cron because you haven't set the right $PATH
makes it hard to track down the problem. Now it says:

    $ ionice -c 3 does-not-exist
    ionice: executing does-not-exist failed: No such file or directory

Signed-off-by: Ævar Arnfjörð Bjarmason <avarab@gmail.com>

schedutils/ionice.c

@@ -184,7 +184,7 @@ int main(int argc, char *argv[])
 			ioprio_setpid(0, ioprio, ioclass);
 			execvp(argv[optind], &argv[optind]);
 			/* execvp should never return */
-			err(EXIT_FAILURE, _("execvp failed"));
+			err(EXIT_FAILURE, _("executing %s failed"), argv[optind]);
 		}
 	}