* Unify reporting of WARNING with log
* Throw exception in functions that are used inside duckdb execution
* Report WARNING for functions that are part of "normal" postgres
  execution
* Cleanup duckdb_state before reporting error in DuckDBNode (clean up)

* Added `PostgresFunctionGuard` and `PostgresVoidFunctionGuard` that
  handle call to Postgres function that can throw ERROR.

* Add few informations how we should handle errors inside code.
* Use same function name `PostgresFunctionGuard` for both void and
  non-void PG function calls.

* Guard function throw `duckdb::Exception(EXECUTOR,...)`
* Updated CONTRIBUTING.md according to review suggestion
* Guarded other suggested pg functions