Revisit error handling in pgsrv

[scsmithr]

For some errors, we propagate up to the server which will then kill the connection. We need to make sure that we limit the kinds of errors that we propagate to avoid disconnecting the client prematurely (e.g. failing to parse a query should not cause a disconnect).

[greyscaled]

Someone just needs to sit down and comb through the handshake and understand each error.

Sometimes you want to close a connection, but others you want to send over the connection to the client and continue.

No amount of grooming should be done, it's just work of going line by line through the code.

[scsmithr]

I'm going to mark this as done. #449 helped in this area, and as far as I can tell, we're sending errors to the client when appropriate.

If there's a case where we don't send back the appropriate error, or if the connection closes when it shouldn't, an issue should be opened up for that specific case.