Report schema merge conflict for incompatible tables created on each branch

If a table is created on branch1 and branch2, but has a different schema, then merge currently aborts with the following error:
```
table with same name 'FOO' added in 2 commits can't be merged
```

This happens because there is no common ancestor commit in both branches that creates the table, so Dolt isn't sure which schema is the correct one to use. Instead of halting the merge with this error message, we should treat this as a schema merge conflict and report it through the standard schema merge conflict table. This would allow users to resolve the schema conflict in the same ways as other schema conflicts and would also enable all schema merge conflicts to be identified up front – instead of fixing one, retrying the merge, potentially hitting this error for another table, etc.