Improve desciption of inverse_map and add option to direclty pass coordi...

See https://github.com/scikit-image/scikit-image/issues/1141
This error confused me a lot, given the statement that nd images are supported. I think this error message should be more explicit about what went wrong. For example "Only 2D images (grayscale or RGB) are supported when providing a non-array inverse map."

In fact, that may be wrong and I'm still confused. Perhaps add a comment to the `else` clause specifying what other possibilities are covered by it?