Refactor error handling for missing foreign key

Moved error handling into DBSchema to homogenize it.
Add support for different operators

Operators added:
=, !=, <>, <, >, <=, >=, null, not_null, in, not_in,
not_distinct_from, distinct_from
Better escaping of arrays

Why is there no lib function for converting php-array to pg-array?!?
Add more comparators

~, ~*, !~, !~*, regexp, like, ilike