Improve error message if shapes disagree

[astrofrog]

Before:

        ...,
        [255, 255, 255],
        [255, 255, 255],
        [255, 255, 255]]], dtype=int16)
actualImage = array([[[255, 255, 255],
        [255, 255, 255],
        [255, 255, 255],
        ...,
        [255, 255, 255],
     ...[255, 255, 255],
        ...,
        [255, 255, 255],
        [255, 255, 255],
        [255, 255, 255]]], dtype=int16)

    def calculate_rms(expectedImage, actualImage):
        "Calculate the per-pixel errors, then compute the root mean square error."
        if expectedImage.shape != actualImage.shape:
            raise ImageComparisonFailure(
                "Image sizes do not match expected size: {} "
>               "actual size {}".format(expectedImage.shape, actualImage.shape))
E           matplotlib.testing.exceptions.ImageComparisonFailure: Image sizes do not match expected size: (153, 177, 3) actual size (146, 170, 3)

/usr/local/lib/python3.5/dist-packages/matplotlib/testing/compare.py:356: ImageComparisonFailure

After:

Error: Image dimensions did not match.
  Expected shape: (800, 600)
    /var/folders/zy/t1l3sx310d3d6p0kyxqzlrnr0000gr/T/tmp9p8eizmu/baseline-test_succeeds.png
  Actual shape: (900, 900)
    /var/folders/zy/t1l3sx310d3d6p0kyxqzlrnr0000gr/T/tmp9p8eizmu/test_succeeds.png

The most important is that the file paths are now included.

Arguably this better error should be in compare_images but that would break the current behavior of raising an exception when the shapes don't match. Also, by adding it here it will work with all versions of Matplotlib.

[astrofrog]

The CI passed, even though the results didn't get reported back here.