stbt.wait_for_match per-frame debug output includes template filename

For long-running `wait_for_match` calls (for example with
`timeout_secs=120`) I find it really annoying to have to scroll up in
the log to find what image it's searching for, and the single line that
says "Searching for <template name>" can be hard to spot among the
hundreds of "No match found" lines. Now each of those "No match found"
lines also says the name of the template image it was searching for (in
addition to the information it already shows: timestamp of the frame,
position of the match, first-pass result).

@wmanley This will conflict with f2c4028e (Allow passing a numpy array
as template) from your pull request #100. Do you want me to hold off on
merging this?