mark_tables_colocated returns unintelligible error message when run against range/append tables

- created to range distributed tables (r1 and r2) with the same schema (a int, b int)
- distributed on the same column (a)
- created an empty shard for each table
- set min/max ranges for each shard to be same (1, 10)
- moved shards to a same node
- run mark_tables_colocated function

got
```
select mark_tables_colocated('r1', ARRAY['r2'::regclass]);
ERROR: cannot colocate tables r1 and r2
DETAIL: Shard intervals don't match for r1 and r2.
```

Related to #1329