Upgrade BeaconError to allow for 200 response code

[jrambla]

Currently, the description of BeaconError is "Beacon-specific error. This should be non-null in exceptional situations only, in which case exists has to be null." I believe that being precise in the values of attributes is desirable, given the ambiguous meaning of "null" ("no" vs "I don't know"). Thus I'll like to update the description to allow for a 200 response code, while keeping "null" also for retrocompatibility, but "not recommended".

[jrambla]

Beacon.md should be also updated to better inform on errors or return codes management.

[sdelatorrep]

I created PR #267