Bug 1729481: Fix a bug in the python-version hacks r=jewilde

I was checking if the caller was 'log' but that wasn't correct
because we pass the result of the function to log, we don't call
it from log

Differential Revision: https://phabricator.services.mozilla.com/D124939

python/mozbuild/mozbuild/vendor/rewrite_mozbuild.py

@@ -255,10 +255,13 @@ def ast_get_source_segment(code, node):
     ):
         return ast.original_get_source_segment(code, node)
 
-    if caller.function == "log":
+    if caller.function == "assignment_node_to_source_filename_list":
         return ""
 
-    raise Exception("ast_get_source_segment is not available with this Python version.")
+    raise Exception(
+        "ast_get_source_segment is not available with this Python version. (ver=%s.%s, caller=%s)"
+        % (sys.version_info.major, sys.version_info.minor, caller.function)
+    )
 
 
 # Overwrite it so we don't accidently use it