Improved messaging for error responses

[Steve-Mcl]

closes #23

Description

Improve messaging and notifications for 4xx errors Permit custom notification text for 403 errors when the body code is "assistant_service_denied" - for feeding back info like "Trial expired"

Related Issue(s)

23

https://github.com/FlowFuse/security/issues/92

Checklist

• [x] I have read the contribution guidelines
• [ ] Suitable unit/system level tests have been added and they pass
• [ ] Documentation has been updated
  • [ ] Upgrade instructions
  • [ ] Configuration details
  • [ ] Concepts
• [ ] Changes flowforge.yml?
  • [ ] Issue/PR raised on FlowFuse/helm to update ConfigMap Template
  • [ ] Issue/PR raised on FlowFuse/CloudProject to update values for Staging/Production

Labels

• [ ] Includes a DB migration? -> add the area:migration label