move socket read error handling logic around a bit

[RalfJung]

We always read when epoll says there is an error waiting. Then read will dispatch to error to get handling suitable for the specific socket. For Tunnel we avoid doing anything (logging or counting the error) for EMSGSIZE.

I realized we are entirely ignoring write errors, so I made them feed the same mechanism.

@mitar is that along the lines of what you had in mind?

[mitar]

Yea. This looks good.