qfile-unpacker: Avoid data loss by checking for child errors

When qfile-unpacker's child encountered an error, it would display an
error message and exit(1), but the parent didn't inspect its status and
exited successfully.

That was unfortunate for qvm-move-to-vm: Even if the destination VM e.g.
didn't have enough free disk space, the RPC call would claim to succeed
anyway, so the file would be deleted from the source VM.

qubes-rpc/qfile-unpacker.c

@@ -90,5 +90,8 @@ int main(int argc __attribute((__unused__)), char ** argv __attribute__((__unuse
 	}
 	if (umount2(".", MNT_DETACH) < 0)
 		gui_fatal("Cannot umount incoming directory");
-	return ret;
+	if (!WIFEXITED(ret)) {
+		gui_fatal("Child process exited abnormally");
+	}
+	return WEXITSTATUS(ret);
 }