GenBank.Scanner: Raise ValueErrors instead of AssertionErrors when parsing fails

Other parts of the parser code all raise ValueError()s when they
encounter unexpected input, just the checks for record.id, record.name
and record.description raise AssertionErrors. Fix this and provide more
meaningful error messages.

For record.name, this code path can be triggered by a GenBank file with
the locus line of "LOCUS        \n".

Signed-off-by: Kai Blin <kai.blin@biotech.uni-tuebingen.de>