[Feature]: Run/fleet plan warnings

[jvstme]

Problem

It is often difficult to tell why the offers you expected to see in the run/fleet plan are missing.

Solution

Introduce run/fleet plan warnings for common cases. An array of warnings can be returned to the client by the server as part of the plan.

Examples of warnings:

• No backends or SSH fleets configured.
• No fleets found (with DSTACK_FF_AUTOCREATED_FLEETS_DISABLED)
• Some of the requested backends are not configured.
• A backend failed to return offers (expired credentials, backend error, etc).
• No instances to --reuse.

Workaround

Follow the server logs, which can help troubleshoot some of the cases.

Would you like to help us implement this feature by sending a PR?

Yes