Show warnings in run plan if some backends fail to return offers

[r4victor]

Currently, if a backend fails to return offers, the run plan won't include them. Users can only see the errors in the server logs. When a user doesn't have access to server logs such as in dstack Sky or a multi-user deployed dstack server, this leads to a suboptimal experience – the errors may go unnoticed when, for example, the backends creds are invalid/insufficient.

The suggestion is to show warnings in run plan if some backends fail to return offers.

Invalid/insufficient creds errors should be handled explicitly in the backend for other operations as well. Currently, they may be treated as no capacity and can only be noticed as warnings in server logs.