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/exceptions.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)