Don't expect exception classes to be available on the top-level object

[jarib]

The specs expect to be able to reference e.g. Watir::Exception::UnknownObjectException as just UnknownObjectException. This is done by including Watir::Exception in the top level object, and frequently leads to NameErrors that pop up in the real world but not in the specs.

We should either (a) fix the specs to reference the full exception name, or (b) only include the Exception module in the specs, not the top level object.