PQsendQuery returned error code

[arifmdev]

This is more or less a re-open of brianc/node-postgres#48, since the error is still occurring. It seems pretty clear, since binding.cc throws when it gets a error result code. Mainly I guess it's more of a question of API, and this seems like a mistake - since the behavior should be to continue to call the callback with an error parameter, and to use PQerrorMessage to get a more descriptive error message.

I might be able to hack up a pull request fixing this, because the error's pretty reproducible from my code.