Add common authentication failure error code

[themiron]

Useful for human-readable logging and can be used for additional logic of custom plugins/further script actions.
For example it's currently used for asuscomm.com in https://github.com/RMerl/asuswrt-merlin.ng

[troglobit]

Great stuff, again!

[themiron]

Many thanks.
NB, only generic "badauth", cloudflare and http 401/403 are handled how.
Guess some of ddns services do have custom auth fail responses, corresponding support can be added later when noticed, not aware of that at the moment

[troglobit]

I noticed. Yeah, now we have the error code at least, so adding it to other plugins where available can be done later 👍