Improve custom privacy request field error reporting

[galvana]

Description

If a SaaS connector is using custom privacy request fields but CONFIG.execution.allow_custom_privacy_request_fields_in_request_execution is disabled we don't receive a very useful error

Unable to replace placeholders in body for the 'None' request of person

This is the error we write out to the logs, but this should be visible from the Admin UI privacy request details

2023-10-12 18:26:32.526 | INFO     |  - Custom fields from privacy request pri_bd9168be-ad82-4222-8e6d-5b2fbafc34ca, but config setting 'CONFIG.execution.allow_custom_privacy_request_fields_in_request_execution' is set to false and prevents their usage.