We ignore link header URLs and just extract the last page number

Not the recommended way to do this but...meh.
Don't use reserved exit codes
Added flag to enable xtrace output for debugging
Refactored _process_response to be reusable

This function can be used to sanity-check all HTTP responses. It's nicer
to not have this coupled to kicking off the issues processing too.
Update _process_issues for changes in _process_response
Added missing quotes around the last_pg variable passed to awk