[Refactor] Remove various clj-ssh related exception handling

The updated Jepsen library relies on scp to upload and download files.

See: https://github.com/jepsen-io/jepsen/blob/6ccf9ce00835faebbea9eb55409a52da8f60ae83/jepsen/src/jepsen/control/scp.clj#L111

This means we can most likely get rid of the alter-var-root of jepsen.control/download that catches the exceptions thrown by clj-ssh.

See: https://github.com/couchbaselabs/jepsen.couchbase/blob/1bab48a83361d6ca69e8cf7ba278df1b2617f327/src/couchbase/core.clj#L179