Gebruik van HTTP statuscode 403 in plaats van 401.

De IG sectie 'Handling errors' en de extra example pagina aanpassen op het gebruik van de HTTP statuscode 403 in plaats van 401 wanneer een request zonder valide autorisatie wordt gedaan.
Daarnaast is de link naar de FHIR spec https://www.hl7.org/fhir/stu3/security.html#AccessDenied toegevoegd in de tekst als onderstaande:
"For the most common errors, the expected handling is listed in table [xxx] below. This should be interpreted as a guideline, as the nature of an error is not always unambiguous and various FHIR reference implementations will make different choices. The FHIR specification provides more detail on error handling in general, access denied responses and for search operations specifically. "

In the FHIR IG section on handling errors, the status code for unauthorized requests has been adjusted from 401 to 403. In the introduction text, a reference to the FHIR security spec has been added.