Previously, BasicErrorController would return the response status
set in the javax.servlet.error.status_code request attribute when
serving JSON but would also return a 200 OK response when serving
HTML. This didn’t cause much trouble when a person was browsing, but
proved problematic for machine clients that request text/html and care
about the response status. For example, the success handler would be
driven for an XHR request even though the response was really an error.

This commit updates BasicErrorController to set the response status for
text/html responses to match the status that it would use in an
application/json response.

Closes gh-4694