Integrity data in Stampede / statistics

We knew this was coming, but the first question form a real user trying out the integrity functionality was:

"I haven't tried digging through the log files, but is there a quick way to see if it caught any integrity errors?"

I believe we have to update Stampede to track not only timings of the integrity checks, but also failures and have them tagged/displayed somehow.

From the top, I would like to see a line in statistics output like:

Pegasus ensured the data integrity of your workflow by doing N integrity checks, taking T time. M issues were detected (which might have handled by retries).