Stringify error response to prevent 'error: [Object]' from appearing in the logs

[crh23]

Issue #, if available:

Description of changes:

Current behaviour: Context broker update lambda logs look like

{
  errors: [
    { entityId: 'urn:ngsi-ld:Vehicle:BD74WHB', error: [Object] },
    { entityId: 'urn:ngsi-ld:Vehicle:BD74WJN', error: [Object] }
  ]
}

New behaviour: logs look like

{
    "errors": [
        {
            "entityId": "urn:ngsi-ld:PublicTransportVehicle:MX62GPJ",
            "error": {
                "type": "https://uri.etsi.org/ngsi-ld/errors/BadRequestData",
                "title": "Bad Request Data.",
                "detail": "Invalid date time format found on observedAt",
                "status": 400
            }
        },
        {
            "entityId": "urn:ngsi-ld:PublicTransportVehicle:SK63AUW",
            "error": {
                "type": "https://uri.etsi.org/ngsi-ld/errors/BadRequestData",
                "title": "Bad Request Data.",
                "detail": "Invalid date time format found on observedAt",
                "status": 400
            }
        }
    ]
}

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