MEDIUM: http-ana: Always report rewrite failures as PRXCOND in logs

MEDIUM: http-ana: Always report rewrite failures as PRXCOND in logs

Rewrite failures in http rules are reported as proxy errors (PRXCOND) in
logs. However, other rewrite errors are reported as internal errors. For
instance, it happens when we fail to add X-Forwarded-For header. It is not
consistent and it is confusing. So now, all rewite failures are reported as
proxy errors.

This patch may be backported if necessary.