Support fail_on_missing_trees option (#580)

* Support fail_on_missing_trees option

Author: ponyisi

docs/query_types.rst

@@ -89,6 +89,8 @@ Each dictionary either has a ``treename`` key (indicating that it is a query on
 
   * ``expressions``, ``cut``, ``filter_name``, ``aliases``: have the same meaning as for `TTree.arrays()`_ in ``uproot``, except that functions aren't permitted (but *glob*\s and *regular expressions*, which are special kinds of strings, are).
 
+  * ``fail_on_missing_trees``: if set to ``True``, will cause the transformation to fail if a tree specified in the query is not present in any of the input files. By default if a requested tree is not present, it will just be ignored and the corresponding output will not be in the output.
+
   Other keys will be ignored.
 
   Most queries will probably use ``filter_names``, which selects specific branches, and ``cut``, which selects specific rows. The ``expressions`` argument permits new values to be computed from the branches in the tree, and ``aliases`` can be used to introduce shorthand to make these expressions cleaner.

servicex/uproot_raw/uproot_raw.py

@@ -1,4 +1,4 @@
-# Copyright (c) 2024, IRIS-HEP
+# Copyright (c) 2024-2025, IRIS-HEP
 # All rights reserved.
 #
 # Redistribution and use in source and binary forms, with or without
@@ -54,6 +54,8 @@ class TreeSubQuery(DocStringBaseModel):
     """Expression to filter the list of considered input variables by type name"""
     aliases: Optional[Mapping[str, str]] = None
     """Define aliases to use in computation and expressions"""
+    fail_on_missing_trees: Optional[bool] = None
+    """Make queries fail if input trees are missing (default False)"""
 
 
 class CopyHistogramSubQuery(DocStringBaseModel):