ENH: Explicitly check for black image and fail informatively.

The problem in #56 was in pims, but trackpy made is confusing by failing inside ``scipy.stats.scoreatpercentile``, like so:

The problem was in pims, but clearly trackpy could be handling this error more nicely:

    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) 

With this PR, we get instead:

    In [3]: tp.locate(black_image, 5)
    ValueError: Image is completely black.