pycaffe: allow unspecified mean. Fixes #671.
pycaffe: reorder exceptions

It doesn't make sense to complain about input not being batch-sized if
it isn't even 4-d in the first place.
pycaffe: test channel_order and input_scale against None

An input_scale of zero should produce warnings/errors/infinities rather
than being the same as an input_scale of one. A channel_order of () or 0
should not be equivalent to the default ordering.