Log the response date in send_to_inbox() errors.

activitypub.c

@@ -128,6 +128,11 @@ int send_to_inbox(snac *snac, char *inbox, char *msg, d_char **payload, int *p_s
     response = http_signed_request(snac, "POST", inbox,
         NULL, j_msg, strlen(j_msg), &status, payload, p_size);
 
+    if (status == 400) {
+        snac_debug(snac, 0, xs_fmt("send_to_inbox error %d (response date: '%s')",
+            status, xs_dict_get(response, "date")));
+    }
+
     xs_free(response);
 
     return status;