How to get the complete error message from T-SQL while error handling?

Unfortunately, the error handling capabilities of SQL Server are limited. When an error occurs, all you can get is the error number, using the @@ERROR global variable. There is no @@ERROR_MESSAGE global variable to get the error description. For a complete error message, you can always query the master..sysmessages table using the error number, but most of these messages have place holders (like %s, %l etc.), and hence we can't get the complete error message.