Be quieter about failing to decode authority digests

This was freaking out some relay operators without good reason, as
it is nothing the relay operator can do anything about anyways.

Quieting this warning suggested by rieo.

src/or/directory.c

@@ -2334,7 +2334,8 @@ client_likes_consensus(networkstatus_t *v, const char *want_url)
       want_len = DIGEST_LEN;
 
     if (base16_decode(want_digest, DIGEST_LEN, d, want_len*2) < 0) {
-      log_warn(LD_DIR,"Failed to decode requested authority digest %s.", d);
+      log_fn(LOG_PROTOCOL_WARN, LD_DIR,
+             "Failed to decode requested authority digest %s.", d);
       continue;
     };