Allow for no exceptions to be raised and cleanup

Allow for no exceptions to be raised

Add some helpful comments

Remove doctest skips

Remove TODO note

Cleanup and skip failing doctest

skimage/_shared/utils.py

@@ -163,4 +163,3 @@ def assert_nD(array, ndim, arg_name='image'):
         ndim = [ndim]
     if not array.ndim in ndim:
         raise ValueError(msg % (arg_name, '-or-'.join([str(n) for n in ndim])))
-