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.