On server side errors (5xx) dump headers

### New feature, improvement proposal

To ease debugging, in case of server side errors (5xx), Resolver should provide option to dump full request and full server response (w/ headers, no content).

This should be done in 1.9.x and 2.x.

Start by tracing how Resolver handles server-side 5xx responses and where request and response diagnostics are produced. Determine how an option could expose full request and response headers without content; done means 5xx failures can emit both header sets when enabled, with behavior covered by the relevant tests.

Written by the indexing model from the issue text.