SERVER-125571 concluded that most CBR tassert-s can be converted to graceful MP fallbacks, avoiding aborting the query. This ticket is the follow-up to implement that.

• Figure out how to do it (likely by throwing an exception of a specific base class & catching it in the plan ranking strategy)
• Modify the existing CBR tassert-s where possible to use that new mechanism.
• Create unit tests to cover these new code paths.