Fix htools/rpc-test after recent changes

Since this program is not built normally, a few recent commits broke
it:

- 707cd3d7 (“Use exitErr instead of explicit error message and
  exitWith”) introduced wrong syntax (missing '$')
- 29a30533 (“Remove unsafePerformIO usage”) changed the signature of
  G.Path.clusterConfFile
- 5183e8be (“Convert query path from string errors to GanetiException”)
  changed the signature of 'getNode'

Signed-off-by: Iustin Pop <iustin@google.com>
Reviewed-by: Michael Hanselmann <hansmi@google.com>