[Enhancement] Clearer LUT format error message in ociobakelut

I incorrectly thought that `ociobakelut` did not support CLF, when in fact the problem was me getting the format specification argument wrong.

The error message said "File format does not support baking" leading me to believe I had specified a format which was valid, but which could not be used for `ociobakelut`.

It would have been clearer if the error had said something like "Unrecognised file format".

Start by locating the ociobakelut code that parses the format specification and emits “File format does not support baking.” Reproduce the invalid-format case, distinguish it from a valid format that cannot be baked, and verify that the resulting error clearly identifies an unrecognised file format.

Written by the indexing model from the issue text.