docs(jans-auth): Error Handling for Person Authentication scripts

[maduvena]

In documentation of Person authentication script, a seperate section on "Error Handling" should be included for the following notes:

Notes:
During authorization request AS records redirect_uri into rp_origin_id cookie.
AS has configuration property errorHandlingMethod. When error occurs during authentication and AS found rp_origin_id it can:

errorHandlingMethod=remote - send error back to RP which gives RP chance to handle error
errorHandlingMethod=internal - show error page (no chance for RP to handle it).
That's why in PR default value is changed to errorHandlingMethod=remote. In addition if during permission check unexpected exception occur we log it and deny request. Also all results from external authn custom script during authentication are logged (independently whether script has print statement or not). This should help understand reason in case of fail requests.

Reference : #4449