chore: standardize missing training config error

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

llama_stack/apis/common/errors.py

@@ -79,3 +79,11 @@ class ConflictError(ValueError):
 
     def __init__(self, message: str) -> None:
         super().__init__(message)
+
+
+class MissingTrainingConfigError(ValueError):
+    """raise when Llama Stack is missing configuration for training"""
+
+    def __init__(self, config_name: str) -> None:
+        message = f"'{config_name}' is required for training"
+        super().__init__(message)