Added existence check before dropping constraints in postgres

The Schema object generates distinct sql queries for altering a table and for removing a constraint or an index. However, in some cases, the `drop` query may come after an `alter table` query deleting the column to which the index/constraint was bound. In postgres, where dropping a column automatically drops its attached indexes and constraints, executing the queries in that order leads to an sql error (trying to drop something that has already been deleted -> "SQLSTATE[42704]: Undefined object").

This PR introduces a quick fix for that issue (not sure it's the best way though).