validator: Handle missing $ref's gracefully

From time to time $ref schemas may be missing due to each schema getting
applied at different times or a change makes the schema invalid (and
testing was ignored). Instead of this resulting in a backtrace splats
return a false schema so only validation fails if used.

Signed-off-by: Rob Herring (Arm) <robh@kernel.org>

Author: robherring

dtschema/validator.py

@@ -423,7 +423,11 @@ def __init__(self, schema_files, id_filter=""):
         self.schemas['version'] = dtschema.__version__
 
     def retrieve(self, uri):
-        return DRAFT201909.create_resource(self.schemas[uri])
+        try:
+            schema = self.schemas[uri]
+        except KeyError:
+            schema = False
+        return DRAFT201909.create_resource(schema)
 
     def annotate_error(self, id, error):
         error.schema_file = id