Better errors for when secrets (POLICY_AUD, etc) are not set

[nbolton]

Depending on what isn't set, you get some mysterious errors that takes a while to figure out by trawling the server logs.

• TEAM_DOMAIN not set -> 401 Authentication error
• POLICY_AUD not set -> 500 error / [Auth] JWT validation failed: unexpected "aud" claim value
• ACCOUNT_ID or API_KEY not set -> API error: 500 / Error: Failed to list databases: 404

Why not check if these are set before doing anything and show an error for each? e.g.

Error: TEAM_DOMAIN env/secret not set

Bonus: As well as the error, link to docs such as: https://developers.cloudflare.com/fundamentals/account/find-account-and-zone-ids/