Feature: Push down connection-checked errors into non-pooled connection code

[Carl Sverre]

Right now, when an error occurs in a pooled connection we test to determine if its a connection exception or not - and if it is we raise PoolConnectionException. The idea of this task is to push that logic down into database.py such that all connections have this helpful exception logic. This is helpful since connections can crash due to a lot of different exceptions - and you usually just care that the connection is down.