libcriu: print error if exec failed

Reported-by: Paschalis Mpeis <paschalis.mpeis@ed.ac.uk>
Signed-off-by: Ruslan Kuprieiev <kupruser@gmail.com>
Signed-off-by: Pavel Emelyanov <xemul@parallels.com>

lib/criu.c

@@ -658,6 +658,7 @@ int criu_restore_child(void)
 		sprintf(fds, "%d", sks[1]);
 
 		execlp("criu", "criu", "swrk", fds, NULL);
+		perror("Can't exec criu swrk");
 		exit(1);
 	}