When there's no .^find_method nor method cache, output debugname

to lessen the amount of confusion caused by this rare,
but often not easy to figure out exception

src/6model/6model.c

@@ -137,8 +137,8 @@ void MVM_6model_find_method(MVMThreadContext *tc, MVMObject *obj, MVMString *nam
             char *c_name  = MVM_string_utf8_encode_C_string(tc, name);
             char *waste[] = { c_name, NULL };
             MVM_exception_throw_adhoc_free(tc, waste,
-                "Cannot find method '%s': no method cache and no .^find_method",
-                 c_name);
+                "Cannot find method '%s' on '%s': no method cache and no .^find_method",
+                 c_name, MVM_6model_get_debug_name(tc, obj));
         }
         else {
             res->o = tc->instance->VMNull;