Improve the error mismatch messages

<thing> was found <count> times in <big long html string> is pretty unhelpful.

We should do better.

Current best idea is to make the search less specific and print things that are close if
we can.

"Tried to match an anchor with href=foo and class=bar, and found an anchor with href=foo
but class=zap"

could be very nice if it gets it right, and printing the whole HTML is always annoying.

Providing the html via addDetails would be spiffy, so it is there if you need it.

Thanks,

James