Use communicationError for network errors

[johto]

This seems cleaner than trying to maintain the list of errors which might have originated from operations on the network socket / TLS layer.

Any thoughts?

[johto]

Though this does change the behaviour a bit. The way we currently deal with network errors only turns a known list of errors into ErrBadConn, which would give the user more information if e.g. the TCPConn returns an error because something's badly wrong.

I wish there was a way to say "this connection is bad", but also specify an error to be returned to the user if database/sql is going to give up on retrying. The way things currently work inside database/sql is pretty ugly.

[tamird]

This seems wrong based on discussion in #422. This change will cause all network errors to bubble up as driver.ErrBadConn, despite the fact that some work may have been done on the server.

[johto]

This seems wrong based on discussion in #422. This change will cause all network errors to bubble up as driver.ErrBadConn, despite the fact that some work may have been done on the server.

That's already what happens. This PR tried to keep the same semantics but without the ugly string comparisons (which, I'm guessing, could fail at any point).

[johto]

I wrote:

I wish there was a way to say "this connection is bad", but also specify an error to be returned to the user if database/sql is going to give up on retrying. The way things currently work inside database/sql is pretty ugly.

Now we actually have the necessary pieces to do this.