These are mostly comments in the code. A few error messages were fixed
and are of low enough importance not to deserve a backport. Some regtests
were also fixed.
2 reg tests are added. The first one ensures the declaration of errors in a
proxy is fonctionnal. It declares http-errors sections and declare error files
using the errorfile and the errorfiles directives, both in the default section
and the frontend sections. The second one ensures it is possible to use a custom
error file for an HTTP deny rule.