[scudo] Descriptive Fucshia errors

`dieOnError` in fuchsia.cpp takes the syscall name, status code, and
relevant size parameter to describe what failed, the reason, and size
context. Several different Fuchsia syscalls can fail underlying Scudo's
map operation and gets hidden if using `dieOnMapUnmapError`.

The reason for this change is to provide an error status code and more
helpful information to debug failures.