switch_root: improve behavior on error; improve help text

*: make "can't execute '%s'" message uniform

shell/cttyhack.c

@@ -73,5 +73,5 @@ int cttyhack_main(int argc UNUSED_PARAM, char **argv)
 	}
 
 	BB_EXECVP(argv[0], argv);
-	bb_perror_msg_and_die("cannot exec '%s'", argv[0]);
+	bb_perror_msg_and_die("can't execute '%s'", argv[0]);
 }