Add detailed error messages about router planner failures

Right now router planner doesn't tell why something failed, we just go on to the distributed planner. That means it's hard to figure out why router failed.

The first step to improve that is to replace all cases where router planner returns NULL with returning an error. Besides being able to print that as a debugging statement, that'll allow us to return good error messages if/when we add a 'router_only' guc (good for migrating apps).

It'd also be good because it improves the error messages in some edge cases (e.g. prepared statments errors via SQL functions)