query executor: better handling of errors for host selection
query executor: better handling of errors for host selection

We cannot mark the host as failed in case of logical errors, whether it is from
gocql itself or from the context package.

mailgun/gocql,Zariel/gocql,vrischmann/gocql,gocql/gocql,gocql/gocql,Zariel/gocql,vrischmann/gocql,mailgun/gocql