makesyscalls: sprinkle some assert() on standard function calls
Improves our error reporting, ensuring that we aren't just ignoring errors in the common case. Note specifically the boundary where we have to change up our error handling approach. It's fine to error() out up until we create the tempdir, then the rest should try to handle it gracefully and abort(). A future change will clean this up further by pcall'ing all of the bits that cannot currently error() without cleaning up.