Complete error handling

* tools/error-code-snarf.scm: clean up output functions; output
  value->error-(code|source) lookup tables; bump copyright

* src/gpg/error-codes.scm.in:
* src/gpg/error-sources.scm.in: use immutable record types; make
  value->error-(code|source) lookup tables auto-generated; bump
  copyright

* src/gpg/errors.scm: complete error handling functionality