[Bug]: No error if backend dependencies are not installed

[jvstme]

Steps to reproduce

1. Install dstack without backend extras

   pip install dstack

2. Configure a backend that does require extras

   > cat ~/.dstack/server/config.yml
   projects:
   - name: main
     backends:
     - type: aws
       creds:
         type: default

3. dstack server

Actual behaviour

dstack ignores the backend silently.

Expected behaviour

dstack displays an error about missing dependencies for the configured backend.

dstack version

0.18.11

Server logs

[18:01:42] INFO     Applying ~/.dstack/server/config.yml...                                                                                                                                    
           INFO     The admin token is *******
           INFO     The dstack server 0.18.11 is running at http://127.0.0.1:3000

Additional information

No response

[github-actions]

This issue is stale because it has been open for 30 days with no activity.

[github-actions]

This issue was closed because it has been inactive for 14 days since being marked as stale. Please reopen the issue if it is still relevant.