pkg-python: improve error handling

Make the python packing a bit easier to use by providing a detailed error
message if <pkg>_SETUP_TYPE isn't set to a valid value.

At the same time adjust the error message used when <pkg>_SETUP_TIME isn't
set to list the exact variable name that should be set, and don't talk about
'Unknown' as it isn't printed when the variable isn't set (or is set to the
empty string).

Signed-off-by: Peter Korsgaard <peter@korsgaard.com>