print the error too if we can't open a directory

It's not intuitive to print

	open ... for domain xyz

it doesn't convey that the open failed.

now it appends the error string, at least the user can understand that
something went wrong.

reported by cage on irc, thanks!

gmid.c

@@ -133,7 +133,8 @@ load_vhosts(void)
 			if (l->dir == NULL)
 				continue;
 			if ((l->dirfd = open(l->dir, O_RDONLY | O_DIRECTORY)) == -1)
-				fatal("open %s for domain %s", l->dir, h->domain);
+				fatal("open %s for domain %s: %s", l->dir, h->domain,
+				    strerror(errno));
 		}
 	}
 }