fix: Fix object has no attribute servers for udfs

[nkitsaini]

We shut down a previously started server if present when starting new python udf server.

    _running_server = AwaitableUvicornServer(config) # <---- we have tracked the server
    # Register the functions only if the app is running interactively.
    if app_config.running_interactively:
        app.register_functions(replace=True) # <----- Something goes wrong here


    asyncio.create_task(_running_server.serve()) # <---- server is started here
    await _running_server.wait_for_startup()

In above code if register_functions fails we will be in a state where we think we have a running server but we don't. This results in unvicorn throwing an error has no attribute 'servers' when we try shutdown this not running server.

I have made two changes to fix this (any one would resolve our issue):

1. Only track _running_server after the function has been registered
2. Run shutdown only after verifying that previous server actually started

Test

Tested by running this notebook.
An easy way to make register_functions fail is to not select a database.

[psachdeva-ss]

LGTM