stbt.MatchTimeout: Avoid verbose numpy representation

...of the template image (in the MatchTimeout exception message) when
the image was given as a numpy array (not a filename).

Commit 2c276e36 added the ability to pass a numpy array as a template
image to `wait_for_match` and other matching functions. But if no match
was found, the output was unnecessarily verbose:

    MatchTimeout: Didn't find match for '[[[255 255 255]
      [255 255 255]
      <snip>
      [176 176 176]]]' within 10 seconds.