[GoldenLion]

Код:

mysqL_errno(MySQL:handle = MYSQL_DEFAULT_HANDLE)

it will return 0 if there were no problems performing the MySQL operation, otherwise it will return the error code.
This is how you can use it after mysql_connect:

Код:

if (mysql_errno()) print("Failed to connect to the database."); // 0 - no errors
else print("Successfully connected to the database."); // there's an error