adding the exception RecordNotFound to UUID find #11957

Basic change to handle UUIDS exception to fix https://github.com/rails/rails/issues/11957 and those related.
Ex. 
User.find(some-non-uuid)
will trigger a RecordNotFound instead of PG::InvalidTextRepresentation
I created an app to test this as well:
https://github.com/joseluistorres/my-test-app

Please provide feedback, thanks!

yes, we would need a second validation like we just did here at the postgresql adapter.