Eager initialisation

Usually a plugin manager will initialise a plugin only when it has to--i.e. when it matches a plugin lookup search and has to return an array of results.

But we can force a plugin to be initialised when it is added to a plugin manager. This is called eager initialisation, because it probably doesn't need to be used yet.

If eager initialisation fails the add() method, which invoked the initialisation, throws an exception. But if initialisation is not eager--i.e. it occurs from a lookup--then failure will be handled quietly and the lookup() method will just not return that failed plugin (although it may return others).