--only-policy option doesn't check policy existence

[gmontard]

Description

Running --only-policy (or --skip-policy) doesn't check if the policy IDs exist, leading to bad UX and misleading output.

Expected Behavior

Throw an error if using a policy ID that doesn't exist

Actual Behavior

Fail silently

Possible Fix

Check if policy IDs exist first

Steps to Reproduce

1. curio scan . --only-policy CR-000
2. curio scan . --skip-policy CR-000

Your Environment

• Version used: 0.21.1
• Target codebase stack (e.g. Ruby): Ruby
• Operating System and version: Mac

[cfabianski]

This also applies to detectors (check the detector exists)