chore: standardize unsupported model error #2517 (#2518)

# What does this PR do?

- 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: remove `register_model`
function implementation from `LiteLLMOpenAIMixin` class. Now uses the
parent class `ModelRegistryHelper`'s function implementation

Closes #2517


## Test Plan


1. Create a new `test_run_openai.yaml` and paste the following config in
it:

```yaml
version: '2'
image_name: test-image
apis:
- inference
providers:
  inference:
  - provider_id: openai
    provider_type: remote::openai
    config:
      max_tokens: 8192
models:
- metadata: {}
  model_id: "non-existent-model"
  provider_id: openai
  model_type: llm
server:
  port: 8321
```

And run the server with:
```bash
uv run llama stack run test_run_openai.yaml
```

You should now get a `llama_stack.exceptions.UnsupportedModelError` with
the supported list of models in the error message.

---

Tested for the following remote inference providers, and they all raise
the `UnsupportedModelError`:
- Anthropic
- Cerebras
- Fireworks
- Gemini
- Groq
- Ollama
- OpenAI
- SambaNova
- Together
- Watsonx

---------

Co-authored-by: Rohan Awhad <rawhad@redhat.com>

llama_stack/providers/utils/inference/model_registry.py

@@ -8,6 +8,7 @@ from typing import Any
 
 from pydantic import BaseModel, Field
 
+from llama_stack.apis.common.errors import UnsupportedModelError
 from llama_stack.apis.models import ModelType
 from llama_stack.models.llama.sku_list import all_registered_models
 from llama_stack.providers.datatypes import Model, ModelsProtocolPrivate
@@ -34,7 +35,9 @@ def get_huggingface_repo(model_descriptor: str) -> str | None:
 
 
 def build_hf_repo_model_entry(
-    provider_model_id: str, model_descriptor: str, additional_aliases: list[str] | None = None
+    provider_model_id: str,
+    model_descriptor: str,
+    additional_aliases: list[str] | None = None,
 ) -> ProviderModelEntry:
     aliases = [
         get_huggingface_repo(model_descriptor),
@@ -81,9 +84,7 @@ class ModelRegistryHelper(ModelsProtocolPrivate):
 
     async def register_model(self, model: Model) -> Model:
         if not (supported_model_id := self.get_provider_model_id(model.provider_resource_id)):
-            raise ValueError(
-                f"Model '{model.provider_resource_id}' is not supported. Supported models are: {', '.join(self.alias_to_provider_id_map.keys())}"
-            )
+            raise UnsupportedModelError(model.provider_resource_id, self.alias_to_provider_id_map.keys())
         provider_resource_id = self.get_provider_model_id(model.model_id)
         if model.model_type == ModelType.embedding:
             # embedding models are always registered by their provider model id and does not need to be mapped to a llama model