Better error message for when model.type is incorrect

In the genai config, when the "type" of the "model" is incorrect, genai will output `RuntimeError: Load model from gemma-3-270m/ failed:Protobuf parsing failed.` without further information. The error message is not reflective of the actual error and should be improved to guide users to reach the correct config.

cc @baijumeswani

Start by reproducing the incorrect model.type configuration in the genai config and tracing where model loading produces the `Protobuf parsing failed` RuntimeError. Identify the existing configuration or model-loading entry point and related error handling. Done means an incorrect type reports a specific, actionable configuration error instead of the misleading protobuf message.

Written by the indexing model from the issue text.