Validation of certificate chains will fail before dcpomatic_setup()

Validation of certificate chains will fail before dcpomatic_setup()
is called, as OpenSSL has not yet been set up.

Make sure that these failures only raise a Bad() (which nobody is yet
listening to) rather than throwing an exception which gets caught
and reported as a failed-to-load config.

If none of the OpenSSL stuff is working chain_valid() will return false
but private_key_valid() will throw an exception (as it tries to get the
leaf certificate, causing a validity check).