Add description to invariant check (#12496)

### Summary & Motivation
Previously this invariant check doesn't have a description so the error
is confusing. Adding a description to make it more clear.

### How I Tested These Changes
```
make black
python -m pytest python_modules/dagster/dagster_tests
```

---------

Co-authored-by: Sean Han <xhan1@flexport.com>

python_modules/dagster/dagster/_core/definitions/time_window_partition_mapping.py

@@ -159,7 +159,10 @@ def _map_partitions(
         to_partitions_def = cast(TimeWindowPartitionsDefinition, to_partitions_def)
 
         if start_offset != 0 or end_offset != 0:
-            check.invariant(from_partitions_def.cron_schedule == to_partitions_def.cron_schedule)
+            check.invariant(
+                from_partitions_def.cron_schedule == to_partitions_def.cron_schedule,
+                "Offset can't be used when cron schedules don't match",
+            )
 
         if to_partitions_def.timezone != from_partitions_def.timezone:
             raise DagsterInvalidDefinitionError("Timezones don't match")