Move enum evhttp_connection_error into http.h from http_struct.h

Because basically if you include http_struct.h then you include http.h
But, not vice versa

So http.h must works without http_struct.h
Drop extra header http_struct.h from regress_http.c
Merge remote-tracking branch 'github-azat-non-deb/pass-http-client-error-codes-to-callback' into 2.1-testing