[REF-2602] if rxconfig.py is not importable reflex fails

[masenf]

Try to import a non-existant module in rxconfig.py and it will say

RuntimeError: Cannot get the app module because `app_name` is not set in rxconfig! If this error occurs in a reflex test case, ensure that `get_app` is mocked.

Instead of showing the ImportError. This is basically un-debuggable

_{From SyncLinear.com | REF-2602}

[jod35]

Facing the same issue with Reflex version 0.4.9