Add getInstance() shortcut methods to Component Script Service

I'm making 2 changes:

• Right now the existing getInstance() methods have package visiblity. This is a mistake introduced in 6525c29 (for commit:
  "XWIKI-8318: Component Script Service should use the Context Component Manager").
• They will return null if the component cannot be looked up
• A getLastError() method is introduced to return any exception raised