RDB Loader: improve updateTable error message

[chuwy]

In a case where a batch has two schemas that would get transformed into the same table, (e.g. some-schema and some_schema) we produce not-so-helpful error message: https://github.com/snowplow/snowplow-rdb-loader/blob/a2bc768803115b15649f9518fc59d30d1b0f50a4/modules/loader/src/main/scala/com/snowplowanalytics/snowplow/rdbloader/db/Migration.scala#L251

At very least we can replace ${s.schema.self.schemaKey.toSchemaUri} with ${current.toSchemaUri}. It would show the table's comment instead of what we already know from state.

Also printed Schema case class looks odd.