chore: standardize model not found error (#2964)

# What does this PR do?
1. Creates a new `ModelNotFoundError` class
2. Implements the new class where appropriate 

Relates to #2379

Signed-off-by: Nathan Weinberg <nweinber@redhat.com>

llama_stack/distribution/routing_tables/common.py

@@ -6,6 +6,7 @@
 
 from typing import Any
 
+from llama_stack.apis.common.errors import ModelNotFoundError
 from llama_stack.apis.models import Model
 from llama_stack.apis.resource import ResourceType
 from llama_stack.apis.scoring_functions import ScoringFn
@@ -257,7 +258,7 @@ async def lookup_model(routing_table: CommonRoutingTableImpl, model_id: str) ->
     models = await routing_table.get_all_with_type("model")
     matching_models = [m for m in models if m.provider_resource_id == model_id]
     if len(matching_models) == 0:
-        raise ValueError(f"Model '{model_id}' not found")
+        raise ModelNotFoundError(model_id)
 
     if len(matching_models) > 1:
         raise ValueError(f"Multiple providers found for '{model_id}': {[m.provider_id for m in matching_models]}")

llama_stack/distribution/routing_tables/models.py

@@ -7,6 +7,7 @@
 import time
 from typing import Any
 
+from llama_stack.apis.common.errors import ModelNotFoundError
 from llama_stack.apis.models import ListModelsResponse, Model, Models, ModelType, OpenAIListModelsResponse, OpenAIModel
 from llama_stack.distribution.datatypes import (
     ModelWithOwner,
@@ -111,7 +112,7 @@ class ModelsRoutingTable(CommonRoutingTableImpl, Models):
     async def unregister_model(self, model_id: str) -> None:
         existing_model = await self.get_model(model_id)
         if existing_model is None:
-            raise ValueError(f"Model {model_id} not found")
+            raise ModelNotFoundError(model_id)
         await self.unregister_object(existing_model)
 
     async def update_registered_models(

llama_stack/distribution/routing_tables/vector_dbs.py

@@ -8,6 +8,7 @@ from typing import Any
 
 from pydantic import TypeAdapter
 
+from llama_stack.apis.common.errors import ModelNotFoundError
 from llama_stack.apis.models import ModelType
 from llama_stack.apis.resource import ResourceType
 from llama_stack.apis.vector_dbs import ListVectorDBsResponse, VectorDB, VectorDBs
@@ -63,7 +64,7 @@ class VectorDBsRoutingTable(CommonRoutingTableImpl, VectorDBs):
                 raise ValueError("No provider available. Please configure a vector_io provider.")
         model = await lookup_model(self, embedding_model)
         if model is None:
-            raise ValueError(f"Model {embedding_model} not found")
+            raise ModelNotFoundError(embedding_model)
         if model.model_type != ModelType.embedding:
             raise ValueError(f"Model {embedding_model} is not an embedding model")
         if "embedding_dimension" not in model.metadata: