Previously, EndpointWebMvcChildContextConfiguration would attempt to
create a /error endpoint, irrespective of whether or not the parent
had such an endpoint. If the endpoint was disabled in the parent this
would cause a failure due to the absence of an ErrorAttributes bean.

This commit updates EndpointWebMvcChildContextConfiguration to make
the creation of its /error endpoint conditional on the existence of
an ErrorAttributes bean.

Closes gh-4164