catch error in clojure-test-mode to prevent user-visible error

The `nrepl-current-connection-buffer` in current CIDER now throws an
error when there is no connection, so this breaks initialization of
`clojure-test-mode`, especially during startup.

This small change fixes this problem
Merge pull request #189 from alexott/master

catch error in clojure-test-mode to prevent user-visible error