Implement API for package build failure reports

[efahl]

openwrt/packages#24324 (comment) item 3

Lets make an API instead of scraping failures.html. The idea is totally cool, but have in mind that this is an updater client -- it may be run on an outdated system which has not been updated for a long time, and depending on ethereal html content is likely to break over time. While some people update their systems daily, others only do that every couple of years or if they see a real need. Both has to work fine.

[efahl]

Collecting references:
https://github.com/openwrt/buildbot/blob/main/phase2/master.cfg#L700