MEDIUM: http-ana: Properly handle internal processing errors

MEDIUM: http-ana: Properly handle internal processing errors

Now, processing errors are properly handled. Instead of returning an error 400
or 502, depending where the error happens, an error 500 is now returned. And the
processing_errors counter is incremented. By default, when such error is
detected, the SF_ERR_INTERNAL stream error is used. When the error is caused by
an allocation failure, and when it is reasonnably possible, the SF_ERR_RESOURCE
stream error is used. Thanks to this patch, bad requests and bad responses
should be easier to detect.