Verify that kgx validate output gets saved

[caufieldjh]

Describe the bug

Transform logs are saved to each output directory - see tsv_transform.log in output like this - but these logs do not appear to contain KGX validate output. Nearly every transform will fail validation in at least some way and it's useful to know these violations.

This first got added in #125 and the expectation there was to write validation output to individual transform tsv_transform.log and to the summary table in stats/validation.tsv`, but the former doesn't seem to happen.