pwiz.py fails for postgres when schemas have cross-dependencies
pwiz certainly has limitations and it's very possible cross-schema foreign keys are one of them. I think I will close this as wontfix but feel free to submit a patch.

I have a large PostgreSQL database, with multiple schemas. Some foreign keys involve cross-schema references. Currently `pwiz` just fails with `KeyError` here and there. In my case I've just quick-fixed script to ignore some missing keys and to spit out `UNDEFINED` instead of missing information, but of course it is not a solution.
The problem is that in my case `pwiz` just failed before outputting code, and was useless. Given the size of DB I prefered to have at least some missing Foreign keys, instead of nothing. It may be fixed temporarily by some `--ignore-errors` key, for example.
