ENH: Explicitly check for black image and fail informatively.

The problem in #56 was in pims, but trackpy made it confusing by failing inside ``scipy.stats.scoreatpercentile``, like so:

    In [1]: black_image = np.zeros((100, 100))

    In [2]: import trackpy as tp

    In [3]: tp.locate(black_image, 5)
    ValueError: operands could not be broadcast together with shapes (0) (2) 

The problem was in pims, but clearly trackpy could be handling this error more nicely. With this PR, we get instead:

    In [3]: tp.locate(black_image, 5)
    ValueError: Image is completely black.