chore: standardize dataset not found error (#2962)

# What does this PR do?
1. Adds a broad schema for custom exception classes in the Llama Stack
project
2. Creates a new `DatasetNotFoundError` class
3. Implements the new class where appropriate 

Relates to #2379

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

llama_stack/apis/common/errors.py

@@ -4,6 +4,11 @@
 # This source code is licensed under the terms described in the LICENSE file in
 # the root directory of this source tree.
 
+# Custom Llama Stack Exception classes should follow the following schema
+#   1. All classes should inherit from an existing Built-In Exception class: https://docs.python.org/3/library/exceptions.html
+#   2. All classes should have a custom error message with the goal of informing the Llama Stack user specifically
+#   3. All classes should propogate the inherited __init__ function otherwise via 'super().__init__(message)'
+
 
 class UnsupportedModelError(ValueError):
     """raised when model is not present in the list of supported models"""
@@ -19,3 +24,11 @@ class ModelNotFoundError(ValueError):
     def __init__(self, model_name: str) -> None:
         message = f"Model '{model_name}' not found. Use client.models.list() to list available models."
         super().__init__(message)
+
+
+class DatasetNotFoundError(ValueError):
+    """raised when Llama Stack cannot find a referenced dataset"""
+
+    def __init__(self, dataset_name: str) -> None:
+        message = f"Dataset '{dataset_name}' not found. Use client.datasets.list() to list available datasets."
+        super().__init__(message)