Handle unknown fw

[dwertent]

Overview

Updating error message in case of unknown framework.

Before:

% kubescape scan framework cis
 ✅  Initialized scanner
 ❌  http-error: '400 Bad Request', reason: 'framework 'cis' not found`

After:

% kubescape scan framework cis
 ✅  Initialized scanner
 ❌  framework 'cis' not found, run `kubescape list frameworks` for available frameworks