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/apis/common/errors.py

@@ -0,0 +1,13 @@
+# Copyright (c) Meta Platforms, Inc. and affiliates.
+# All rights reserved.
+#
+# This source code is licensed under the terms described in the LICENSE file in
+# the root directory of this source tree.
+
+
+class UnsupportedModelError(ValueError):
+    """raised when model is not present in the list of supported models"""
+
+    def __init__(self, model_name: str, supported_models_list: list[str]):
+        message = f"'{model_name}' model is not supported. Supported models are: {', '.join(supported_models_list)}"
+        super().__init__(message)