When an error occurs in the serializer, logException() is typically called. The error message, line number and other information is typically not available for use by tooling. WTKXSerializer should provide a structured object with the error information in it e.g. the line number.
I don't have a patch for this but I think a simple error object and a property accessor would be all that's needed. The error object could be accessed as part of exception handling.
I came across when writing pivotpad (xamlpad like tool) and wanted to highlight the error in the bxml text.
This improvement is targeted towards tooling support and has no other apparent use.