spawn: reduce 2 lines, change fprintf() + perror() + exit() to die("... :")

when calling die and the last character of the string corresponds to
':', die() will call perror(). See util.c

Also change EXIT_SUCCESS to EXIT_FAILURE

dwm.c

@@ -1653,9 +1653,7 @@ spawn(const Arg *arg)
 			close(ConnectionNumber(dpy));
 		setsid();
 		execvp(((char **)arg->v)[0], (char **)arg->v);
-		fprintf(stderr, "dwm: execvp %s", ((char **)arg->v)[0]);
+		die("dwm: execvp '%s' failed:", ((char **)arg->v)[0]);
-		perror(" failed");
-		exit(EXIT_SUCCESS);
 	}
 }