Tidy up journal writer code comments and error handling

[fulghum]

Before this change, the code that iterates over journal records would silently stop processing journal records once it encountered any invalid record. This change turns those invalid records into errors so that data isn't silently ignored. Customers can opt-in to the previous behavior by setting the DOLT_SKIP_INVALID_JOURNAL_RECORDS environment variable to any value.

[zachmu]

This seems fine, but maybe @reltuk should comment on whether there was a good reason for the old behavior. Would hate for this to start dying on an edge case we aren't aware of.

[coffeegoddd]

@fulghum DOLT

comparing_percentages
100.000000 to 100.000000

version	result	total
f06b307	ok	5937457

version	total_tests
f06b307	5937457

correctness_percentage
100.0