Modify "not ok" behavior

[perrygeo]

The purpose of raster-tester is to ask yes/no questions about raster data. If the answer is no, that's not an exception, it's just one of the possible expected outcomes. We should specify this behavior in more detail to standardize it.

The following is just a proposal, would love to get feedback/discussion before we start implementing.

If the answer to the question is no:

• the function should return a tuple with boolean and a message like (False, "These rasters are bad and they should feel bad")
• the click command should print the message (to stderr or stdout?), along with the rasters that were being evaluated, not ok: a.tif vs b.tif (These rasters are bad and they should feel bad)
• the click command should exit with code 1

If the answer to the question is yes:

• the function should return a tuple with boolean and a message like (True, "These rasters are perfect in every way")
• the click command should print the message to stdout, along with the rasters that were being evaluated, ok: a.tif vs b.tif (These rasters are perfect in every way)
• the click command should exit with code 0

cc @sgillies @dnomadb