BUG/MINOR: filters: Properly set the HTTP status code on analysis error

BUG/MINOR: filters: Properly set the HTTP status code on analysis error

When a filter returns an error during the HTTP analysis, an error must be
returned if the status code is not already set. On the request path, an error
400 is returned. On the response path, an error 502 is returned. The status is
considered as unset if its value is not strictly positive.

If needed, this patch may be backported to all versions having filters (as far
as 1.7). Because nobody have never report any bug, the backport to 2.0 is
probably enough.