MINOR: http-rules: Handle all message rewrites the same way

MINOR: http-rules: Handle all message rewrites the same way

In HTTP rules, error handling during a rewrite is now handle the same way for
all rules. First, allocation errors are reported as internal errors. Then, if
soft rewrites are allowed, rewrite errors are ignored and only the
failed_rewrites counter is incremented. Otherwise, when strict rewrites are
mandatory, interanl errors are returned.

For now, only soft rewrites are supported. Note also that the warning sent to
notify a rewrite failure was removed. It will be useless once the strict
rewrites will be possible.