An on_load function is supposed to return 'true' to indicate
that the module should be loaded, and 'false' if it should be
unloaded. But returning any other term, as well as causing an
exception, will also unload the module.
Since we don't like boolean values mixed with other values,
change the expected return value as follows:
* If 'ok' is returned, the module will remain loaded and become
callable.
* If any other value is returned (or an exception is generated),
the module will be unloaded. Also, if the returned value is
not an atom, send a warning message to the error_logger
(using error_logger:warning_msg/2).
The new interpretation of the return value means that an on_load
function can now directly return the return value from
erlang:load_nif/2.