Introduce a query settings fallback reason field in the explain response, denoting why the fallback mechanism was engaged. This should only be visible if:

• Query Settings are present on the given query.
• The application of said settings caused an non-ok status, and the fallback mechanism was engaged.

This reason can be bubbled up from get_executor.cpp prepare() call. Also, considering that explain commands bypass the plan cache, it's safe to assume that each explain call will go through planning.