Fix exception swallowing in CompositePluginManager.start.

The `CompositePluginManager.start` method swallows exceptions, leading to an inscrutable "TypeError: argument 2 to map() must support iteration" message (for example when iterating through a `PluginManager` fails due to an `ImportError`).  The exception swallowing is the result of http://bugs.python.org/issue11655.  It's easily avoided by replacing the (abominable, IMO :-) `map` uses in `CompositePluginManager.start`.  For good measure, I've made the same change in `CompositePluginManager.stop`.