Don't try to switch namespace if we don't know which one to switch to.

- `clojure-find-ns` is sometimes unable to figure out what the current
  namespace is. If that's the case, don't try to switch (avoids nasty
  exception) and show a friendly message
Merge pull request #340 from ragnard/fix-nrepl-set-ns-with-unknown-ns

Don't try to switch namespace if we don't know which one to switch to.