This is a partial revert of 582ded6a to
restore the assumed onerror=continue behavior for ESI includes, unless
the feature flag esi_include_onerror is raised.

The part of the change that considers all status codes besides 200 and
204 to be errors for ESI includes remains. A test case covers VCL's
ability to "bless" error responses by overriding resp.status, allowing
ESI delivery to continue on this criterion.

Fixes #4053