Improve commit timestamp is older than oldest timestamp error message

When a transaction tries to commit with a timestamp older than the oldest timestamp, the following error message is given:

WiredTiger error (22) [1511963818:335459][37495:0x7f83dd92e9c0], WT_SESSION.timestamp_transaction: commit timestamp 3E800000002 older than oldest timestamp: Invalid argument

It would be convenient if that error message also included what WiredTiger used for the oldest timestamp. I'm not suspecting any WT bugs; rather the first thing I have to do after seeing that error is rerun the test with more logging.