Fix DBIngestor calls in load_from_database , it was never actually reaching the db.
execute_query/export_table need the connection string as their first arg,
but we were only passing it to the constructor's config dict, which
those methods don't read. Every call blew up with a TypeError before
connecting.
Also split the ImportError/OSError handling , they were caught together
so a real connection failure got reported as "module not available",
which sent people looking in the wrong place. OSError now surfaces as
an actual failure with the original exception chained via `from e`.
Fixes#973.
yzxcj797