Allow custom errors to be nested so that information from more specific errors is not lost

[MLenterman]

We use BuildError.xsl to build custom errors. Currently higher level errors will not retain any information from lower level errors, while the lower level errors usually contain the most useful and specific information.

The BuildError.xsl should be expanded to have an additional element called "cause". If the pipe input is a custom error, it should be copied into the cause element.