miniscript: convert non-critical asserts to CHECK_NONFATAL

The Miniscript code contains assertions to prevent ending up in an insane state or prevent UB, but also to enforce logical invariants. For the latter it is not necessary to crash the program if they are broken. Raising an exception suffices, especially as this code is often called through the RPC interface which can in turn handle the exception and the user can report it to developers.

This revives #28678 from Pieter Wuille.