Improve error message when agent process has errors

Not setting the model API key results in an error in the agent process which is not propagated to the user in readable fashion. It is also not clear how the user should set this API key.

<img width="1917" height="132" alt="Image" src="https://github.com/user-attachments/assets/b5e438e2-c88d-4d00-a679-01a0cb504240" />