Enhance validator output to make filtering by error type more straightforward

[DavidUnderdown]

Experience of using the validator in live projects suggests that it is often useful to be able to concentrate on one error type at a time. If output was more structured it would be easier to import data into other programs (eg Excel), and use the filtering options available in there to examine error types one-by-one to understand the underlying issues. This is especially true for transcription data where there are often different systemic issues affecting the transcription of each individual field.

Suggest at least a CSV output format, but could consider XML too.

[mhGLEIF]

This is definitely needed to use at scale and / or programmatically.