ukify: check early if inputs exist and are readable

ukify: check early if inputs exist and are readable

It's much nicer for the user if we fail early instead of doing partial
processing if we cannot read some input. We can't do those checks immediately
from argparse.Parser.parse_args(), because we want to fully process the
commandline first. In particular, even with invalid args, if --help is
specified somewhere, we want to handle that. Thus, we need to delay the checks
after argparse.Parser.parse_args() returns.

Ukify didn't have type annotations on functions, but it probably should.
Jörg's suggested correction included them and we might just as well start here.