Improve error detection/reporting in Catalog.pm and genbki.pl.

Improve error detection/reporting in Catalog.pm and genbki.pl.

Clean up error messages relating to mistakes in .dat files: make sure they
provide the .dat file name and line number, not the place in the Perl
script that's reporting the problem.  Adopt more uniform message phrasing,
too.

Make genbki.pl spit up on unrecognized field names in the input hashes.
Previously, it just silently ignored such fields, which could make a
misspelled field name into a very hard-to-decipher problem.  (This is in
genbki.pl, *not* Catalog.pm, because we don't want reformat_dat_file.pl to
complain about unrecognized fields.  We'd rather it silently dropped them,
to facilitate removing unwanted fields after a reorganization.)