improve exec failure message

Reference: /n/atom/patch/applied/usbexecfail
Date: Wed May 14 18:12:13 CES 2014
Signed-off-by: quanstro@quanstro.net

--- /sys/src/cmd/usb/usbd/dev.c	Wed May 14 18:12:13 2014
+++ /sys/src/cmd/usb/usbd/dev.c	Wed May 14 18:12:13 2014
@@ -175,7 +175,7 @@
 			argv[0] = fname;
 			xexec(sa->rc, fname, argv);
 		}
-		fprint(2, "%s: %s: not found. can't exec\n", argv0, dt->name);
+		fprint(2, "%s: %s: exec: %r\n", argv0, dt->name);
 		sendul(sa->rc, -1);
 		threadexits("exec");
 	}else{