lib/repo.c: add user-facing error message when repodata doesn't exist

Instead of just cryptically exiting 0 when opening a non-existant
repodata or stagedata, with the only message existing in the debug
output, show an error to the user on stderr.

mentioned in #519

lib/repo.c

@@ -246,6 +246,7 @@ repo_open_with_type(struct xbps_handle *xhp, const char *url, const char *name)
 		int rv = errno;
 		xbps_dbg_printf("[repo] `%s' open %s %s\n",
 		    repofile, name, strerror(rv));
+		xbps_error_printf("%s: %s\n", strerror(rv), repofile);
 		goto out;
 	}
 	if (repo_open_local(repo, repofile)) {