source-sqlserver: Validate table existence and not just CDC instances

[willdonnelly]

We should check during prerequisite validation whether a table exists, and not just whether it has a CDC instance. Presumably the intent was that the existence check is redundant because you can only have a CDC instance if the table exists, but telling the user "table foo has no capture instances and we couldn't create one" is confusing and unhelpful when the real issue is that they dropped that table, so we should check for table existence separately and give a more useful error message in those cases.