confusing error message for failed transaction with constraint violations

When committing a transaction, dolt merges the session's changes into what's already there. If another concurrent transaction makes changes that, when merged with this session's, will result in a constraint violation, we throw this error:

```
Committing this transaction resulted in a working set with constraint violations, transaction rolled back. This constraint violation may be the result of a previous merge or the result of transaction sequencing. Constraint violations from a merge can be resolved using the dolt_constraint_violations table before committing the transaction. To allow transactions to be committed with constraint violations from a merge or transaction sequencing set @@dolt_force_transaction_commit=1.
```

The advice about resolving constraint violations is not applicable to the concurrent use case and is unhelpful and confusing. It's there because the same validation code happens on the `call dolt_merge()` path, where that advice applies. We should have two different tailored error messages for these two different scenarios.