Handle exceptions from get_offers()

[r4victor]

Currently, dstack server does not handle exceptions raised by backends in get_offers():

dstack/src/dstack/_internal/server/services/backends/__init__.py

Line 300 in 78e99d6

tasks = [run_async(backend.compute().get_offers, requirements) for backend in backends]

This means that if one of the backends is not available, the server returns 500. This should be fixed by not including the offers of a broken backend and logging the exception to the server log. Later, we may consider passing the error info to the CLI.