Corrected test module docstring
Corrected wrong export property in dispatcher test
Dispatcher & Imports Registry use the "if_valid" flag in decorators

Avoids to handle a validation flag in the code
Clean up factory bundle context on unregistration

When a component factory is unregistered, the bundle context it is
associated to must be cleaned up to avoid references to a previous
framework instance.
Raise a RemoteServiceError when an endpoint is unknown

... instead of a KeyError