The specifications for functions that load code in the 'code'
module (e.g. code:load_file/1) have some problems:

* The specs claim that the functions can return {error,on_load}, but
  they never do. However, they can return {error,on_load_failure} if
  the -on_load function in a module fails.

* The specs claim that the functions can return {error,native_code},
  but they never do.

While we are it, also extend the on_load_errors/1 test case to test
that the load functions return {error,on_load_failure} when an
-on_load function fails.