Detecting and Processing Errors in XML Queries and Updategrams

How can I detect and display errors that occur during updategram processing?

Let's approach this question by examining the general topic of detecting errors, then looking at an updategram example. When you use SQL Server 2000's XML functionality, SQL Server generates a special XML construct called a processing instruction (PI) to signal an error to the application that processes the XML data. The PI includes the error code, the source of the error, and a description of the error. To see how SQL Server returns errors, let's look at a query embedded in an XML template.