[Bug]: Unexpected error if system Python version is unknown

[jvstme]

dstack version

master

Steps to reproduce

1. Install dstack and run dstack server on a Python version not listed in PythonVersion
2. Run a job without specifying python or image in config file

   $ cat hello.yml
   type: task
   
   commands:
     - python -c 'print("Hello dstack!")'
   
   $ dstack run . -f hello.yml
   

Expected result

The user is notified that they should specify the Python version in config file, as their system Python version is not supported

Actual result

CLI:

Unexpected dstack server error

Server logs:

ValueError: '3.12' is not a valid PythonVersion