Returns the reason of the last failure of polite_request(),
polite_get() or download_pdf() in this session. Every new request
resets it, so read it just after the call that returned NULL.
Details
Replaces the option cb.last_failure of central_bank_database, which the
collectors had to reset by hand. Reasons can be sorted by failure_kind().
See also
Other politeness:
check_robots(),
polite_request(),
request_count()