Improve error handling

[OlivierChirouze]

Error handling must be improved, at least with the following:

• use error enums end to end (operator, operator proxy, frontend)
• return errors from PAF-lib so that they can be handled correctly by UIs (today it only returns data)
• avoid exception throwing and improve logging

https://github.com/prebid/paf-mvp-implementation/pull/120/files/5fb8a059d1d70fa226c13aba8c9655baad2ef549#diff-21193811b758f6d40b39e174d4b5622f8b741a520255c5351f18056bf93fe4c8R452