db_lmdb: log which output was not found in get_output_key

src/blockchain_db/lmdb/db_lmdb.cpp

@@ -2499,7 +2499,8 @@ output_data_t BlockchainLMDB::get_output_key(const uint64_t& amount, const uint6
   MDB_val_set(v, index);
   auto get_result = mdb_cursor_get(m_cur_output_amounts, &k, &v, MDB_GET_BOTH);
   if (get_result == MDB_NOTFOUND)
-    throw1(OUTPUT_DNE("Attempting to get output pubkey by index, but key does not exist"));
+    throw1(OUTPUT_DNE(std::string("Attempting to get output pubkey by index, but key does not exist: amount " +
+        std::to_string(amount) + ", index " + std::to_string(index)).c_str()));
   else if (get_result)
     throw0(DB_ERROR("Error attempting to retrieve an output pubkey from the db"));