🐛 show error when unauthenticated

[czunker]

'cnspec vuln' now shows an error when an unathenticated request is made.

Fixes #629

[czunker]

Before this PR, we already handled the case for an empty config:

cnspec vuln container image photon:4.0-20221224 --config /dev/null                                                                                                            
→ loaded configuration from /dev/null using source --config
FTL vulnerability scan requires authentication, future versions will not have this restriction, login with `cnspec login --token`

This PR shows the same error when using the wrong config:

cnspec vuln container image photon:4.0-20221224 --config /tmp/sa-defect.json                                                                                                           
→ loaded configuration from /tmp/sa-defect.json using source --config
→ using service account credentials
→ discover related assets for 1 asset(s)
→ resolved assets resolved-assets=1
FTL vulnerability scan requires authentication, future versions will not have this restriction, login with `cnspec login --token`

[chris-rock]

Thank you @czunker