Improve handling of signals and 'throw' in modules

Improve handling of signals and 'throw' in modules

* src/emacs-module.c: Add commentary explaining how to write
functions in this file.
(module_make_global_ref, module_free_global_ref)
(module_non_local_exit_signal, module_non_local_exit_throw)
(module_make_function, module_funcall, module_intern)
(module_type_of, module_is_not_nil, module_eq)
(module_extract_integer, module_make_integer)
(module_extract_float, module_make_float)
(module_copy_string_contents, module_make_string)
(module_make_user_ptr, module_get_user_ptr, module_set_user_ptr)
(module_get_user_finalizer, module_set_user_finalizer)
(module_vec_set, module_vec_get, module_vec_size)
(module_non_local_exit_signal_1, module_non_local_exit_throw_1):
Do nothing and return with failure indication immediately, if some
previous module call signaled an error or wants to throw.  See
http://lists.gnu.org/archive/html/emacs-devel/2015-11/msg02133.html
for the relevant discussions.