Replace panics with proper errors when a valid plan does not exist.
Currently, this never happens because a naive plan is always available.
However, once virtual tables are integrated into the planner, it may
occur—for example, when table-valued function arguments are column
references, and the function cannot be placed in the join order so that
its arguments can be evaluated.
Although this change is effectively a no-op for now, it is extracted
into a separate commit to avoid polluting the one that introduces
virtual table integration with the planner.
521eb2368e