chore: standardize vector store not found error (#2968)

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

Relates to #2379

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

llama_stack/apis/common/errors.py

@@ -26,6 +26,14 @@ class ModelNotFoundError(ValueError):
         super().__init__(message)
 
 
+class VectorStoreNotFoundError(ValueError):
+    """raised when Llama Stack cannot find a referenced vector store"""
+
+    def __init__(self, vector_store_name: str) -> None:
+        message = f"Vector store '{vector_store_name}' not found. Use client.vector_dbs.list() to list available vector stores."
+        super().__init__(message)
+
+
 class DatasetNotFoundError(ValueError):
     """raised when Llama Stack cannot find a referenced dataset"""