Add `no-error` optional argument to `nrepl-current-connection-buffer`
[Fix #814] Use `cider-currnt-ns` instead of `nrepl-buffer-ns`

Other changes:

   - Don't reset `nrepl-buffer-ns` in default response handler.

   - Remove `cider-find-ns` (it's confusing to have two functions doing the same
     thing).

   - Use nil global value of `nrepl-buffer-ns`. This will allow easy detection
     of cases when this variable is used but shouldn't.