Increase safety checks in initialisation code

[Ivan-Velickovic]

While integrating the FATFS code I have found it easy to make hard-to-debug mistakes with the initialisation of cothreads, particularly around either getting stack pointers wrong or passing in the wrong number of stacks for how many cothreads exist.

These issues can be easy to make but quite difficult to debug, it would help to have some more debug checks that validate that the stacks provided are accessible and don't cause faults etc.