Ensemble hook api should have better error checking/validation of args

Clint posted an example on irc of a traceback he got from a hook..

2011-03-17 20:14:44,133: twisted@ERROR: Unhandled Error
Traceback (most recent call last):
  File "/usr/lib/python2.6/dist-packages/twisted/internet/defer.py", line 307, in errback
    self._startRunCallbacks(fail)
  File "/usr/lib/python2.6/dist-packages/twisted/internet/defer.py", line 354, in _startRunCallbacks
    self._runCallbacks()
  File "/usr/lib/python2.6/dist-packages/twisted/internet/defer.py", line 371, in _runCallbacks
    self.result = callback(self.result, *args, **kw)
  File "/usr/lib/python2.6/dist-packages/twisted/internet/defer.py", line 879, in gotResult
    _inlineCallbacks(r, g, deferred)
--- <exception caught here> ---
  File "/usr/lib/python2.6/dist-packages/twisted/internet/defer.py", line 821, in _inlineCallbacks
    result = result.throwExceptionIntoGenerator(g)
  File "/usr/lib/python2.6/dist-packages/twisted/python/failure.py", line 338, in throwExceptionIntoGenerator
    return g.throw(self.type, self.value, self.tb)
  File "/usr/lib/ensemble/ensemble/ensemble/hooks/protocol.py", line 125, in get
    data = yield context.get(unit_name)
  File "/usr/lib/python2.6/dist-packages/twisted/internet/defer.py", line 821, in _inlineCallbacks
    result = result.throwExceptionIntoGenerator(g)
  File "/usr/lib/python2.6/dist-packages/twisted/python/failure.py", line 338, in throwExceptionIntoGenerator
    return g.throw(self.type, self.value, self.tb)
  File "/usr/lib/ensemble/ensemble/ensemble/state/hook.py", line 121, in get
    unit_id = yield self._resolve_name(unit_name)
  File "/usr/lib/python2.6/dist-packages/twisted/internet/defer.py", line 823, in _inlineCallbacks
    result = g.send(result)
  File "/usr/lib/ensemble/ensemble/ensemble/state/hook.py", line 88, in _resolve_name
    unit_id = self._topology.get_service_unit_id_from_name(unit_name)
  File "/usr/lib/ensemble/ensemble/ensemble/state/topology.py", line 203, in get_service_unit_id_from_name
    service_name, unit_sequence_id = unit_name.split("/")
exceptions.ValueError: need more than 1 value to unpack

It looks like an invalid unit name was passed to relation-get.. Ideally relation-get and all of the hook api would be doing some better validation of cli args, before it gets that far into the ensemble stack.