Allow StartPlacementConnection() to error out on invalid placementId

#1133 / #1138 showed it'd be useful if StartPlacementConnection() asserted if an invalid placement id were specified. Unfortunately that's not possible right now, because the fake task router planner generates for shardless queries, use a fake placement.

There's various possible fixes to this:
- don't generate fake task placements, just allow task-less queries
- use a node connection for placement-less queries
- continue like now
- ?