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

[Masen Furer]

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

• If no config, tell them to init (should already be working)
• need to catch on the get_config line and raise those exceptions

_{From SyncLinear.com | REF-2602}

[Ssali Jonathan Kiggundu]

Facing the same issue with Reflex version 0.4.9