Introduce Exception types

[iamed2]

All the error calls could be different exception types. In addition, exception types representing PostgreSQL errors should contain a reference to the connection, to enable follow-up actions (including printing more verbose errors).

[oxinabox]

This would be really nice.
It would make it easier to retry on

ErrorException("server closed the connection unexpectedly
    This probably means the server terminated abnormally
    before or while processing the request.\n"),
    Any[(error at loggers.jl:429, 1), (#handle_new_connection#8 at LibPQ.jl:155, 1), 

I guess it means regexing the error messages?

[iamed2]

In #113 this would be LibPQ.ConnectionFailure