Develop Java ErrorListener for better integration with Saxon XSLT Processor

[pgfearo]

Currently the Saxon XSLT Processor command-line stdout is parsed, this is fragile as all error cases are most likely not covered. The text structure sent to stdout is also subject to change.

From the Saxon command-line it's possible to set the -errorListenerClass. Providing a dedicated error listener would improve robustness and also handle a known issue when referencing imported/included XSLT files not in the same folder as the entry XSLT