Better error message for VARCHAR vs VARCHAR cross shard queries

[tirsen]

If you do a cross shard query where you're ordering or grouping by a VARCHAR column you will get the following error message from Vitess:

Caused by: java.util.concurrent.ExecutionException: java.sql.SQLRecoverableException: legacy_code: NOT_IN_TX_LEGACY
message: "vtgate: http://d100db6e568c:27000/: transaction rolled back due to partial DML execution: types are not comparable: VARCHAR vs VARCHAR"
code: ABORTED

This is nearly incomprehensible. We should catch this at a nice point and rethrow it with a clearer error message that explains how to resolve.

There's 3 workarounds:

1. Make it not cross shard,
2. change column to VARBINARY,
3. set column type in vschema.

This also happens for ENUM columns.

For those that are interested, the reason this doesn't work is because Vitess doesn't have support for all MySQL collations so they can't emulate exactly the order by/group by rules properly in vtgate for cross shard queries.