genbki stricter error handling

genbki.pl says:

# Perform OID lookups on an array of OID names.
# If we don't have a unique value to substitute, warn and
# leave the entry unchanged.
# (A warning seems sufficient because the bootstrap backend will reject
# non-numeric values anyway. So we might as well detect multiple problems
# within this genbki.pl run.)

This is fine, but I have found this to be a bit cumbersome in practice
sometimes, because errors are then not easily seen at build time but
have to be extracted from some log files during test runs.

I propose the attached patch to make genbki.pl error out if it
encounters any errors in this routine, while preserving the property
that all errors in one run are reported.