Before this commit, this was defined to trigger undefined behavior. This
was nice because it required less code; but on the other hand, Lua as
well as IPC support had to check these things manually. Do it directly
in the API to avoid code duplication, and to make the API more robust.
(The total code size still grows, though...)
Since all of the failure cases were originally meant to ruin things
forever, there is no way to return error codes. So just print the
errors.
756adee999