[script] Add an iPXE error URI to the "not in a script" message

Signed-off-by: Michael Brown <mcb30@ipxe.org>

src/image/script.c

 
 	/* Sanity check */
 	if ( ! script ) {
-		printf ( "Not in a script\n" );
-		return -ENOTTY;
+		rc = -ENOTTY;
+		printf ( "Not in a script: %s\n", strerror ( rc ) );
+		return rc;
 	}
 
 	/* Parse label */