[UX] Make "No fleets" run status more explicit

[peterschmidt85]

Problem:
Currently No offers is shown in both cases, when dstack can't find matching offers and when no fleets found. Those are two totally different situations that require different action.

Solution:

1. Warning
On dstack apply, if the run plan has no offers, check if fleets exist, and if no fleets exist, instead of

No matching instance offers available. Possible reasons: https://dstack.ai/docs/guides/troubleshooting/#no-offers

Print:

The project has no fleets. Create one before submitting a run: https://dstack.ai/docs/concepts/fleets"

2. Status
In case job termination reason is no capacity and termination reason message starts with No fleet found, in dstack ps and UI, instead of no offers, show no fleets.