config: Reject rather than silently ignoring wrong closure values

config: Reject rather than silently ignoring wrong closure values

find_cl_if(...,fail_if_invalid=False,,..) is only called in places
where a particular closure, or nothing, is expected.

It is really not a good idea for this function to silently ignore
config keys of the wrongn type.  It should behave more like
dict_read_number and dict_read_bool.

There are possible compatibility implications, but they can be solved
by deleting the wrong (currently-ignored) information from the config
file(s).

Signed-off-by: Ian Jackson <ijackson@chiark.greenend.org.uk>