Error catching for internal tasks (queries)

[linmagit]

We added support for internal tasks in #1502, but we don't catch the errors when the internal tasks fail. Instead, we just print our error log messages from the command line. We should discuss how we really want to handle these errors.

[linmagit]

Based on the discussion during today's team meeting, we're going to return the internal task execution status (e.g., success/failure) through a Future and let the caller decide how to handle that. So I think our infrastructure for DML tasks already supports that, but we should also support that for the DDL tasks, e.g., during the DBMS start time to create the internal tables.