chore: standardize unsupported model error #2517

- llama_stack/exceptions.py: Add UnsupportedModelError class
- remote inference ollama.py and utils/inference/model_registry.py:
Changed ValueError in favor of UnsupportedModelError
- utils/inference/litellm_openai_mixin.py: remote register_model func.
Now uses parent class ModelRegistry's func

Closes #2517

llama_stack/providers/utils/inference/litellm_openai_mixin.py

@@ -40,7 +40,6 @@ from llama_stack.apis.inference.inference import (
     OpenAIMessageParam,
     OpenAIResponseFormatParam,
 )
-from llama_stack.apis.models.models import Model
 from llama_stack.distribution.request_headers import NeedsRequestProviderData
 from llama_stack.log import get_logger
 from llama_stack.providers.utils.inference.model_registry import ModelRegistryHelper
@@ -91,12 +90,6 @@ class LiteLLMOpenAIMixin(
     async def shutdown(self):
         pass
 
-    async def register_model(self, model: Model) -> Model:
-        model_id = self.get_provider_model_id(model.provider_resource_id)
-        if model_id is None:
-            raise ValueError(f"Unsupported model: {model.provider_resource_id}")
-        return model
-
     def get_litellm_model_name(self, model_id: str) -> str:
         # users may be using openai/ prefix in their model names. the openai/models.py did this by default.
         # model_id.startswith("openai/") is for backwards compatibility.