workload/schemachange: enhance logging during operation generation

Currently, when generating an operation in the schema changer workload,
we don't have enough information to diagnose failed screening queries.
This patch wraps errors during query generation to track which operator
we were currently working on.

Release note: None

pkg/workload/schemachange/operation_generator.go

@@ -216,7 +216,7 @@ func (og *operationGenerator) randOp(
 			if errors.Is(err, ErrSchemaChangesDisallowedDueToPkSwap) {
 				continue
 			}
-			return nil, err
+			return nil, errors.Wrapf(err, "failed generating operation: %s", op)
 		}
 
 		// Screen for schema change after write in the same transaction.