py: Include filename in errors from loading/saving files via "open".

This improves error messages in mpy-cross:
- When loading a .py file that doesn't exist (or can't be opened) it now
  includes the filename in the OSError.
- When saving a .mpy file that can't be opened it now raises an exception
  (prior, it would silently fail), and includes the filename in the
  OSError.

Signed-off-by: Damien George <damien@micropython.org>

py/reader.c

@@ -139,7 +139,7 @@ void mp_reader_new_file(mp_reader_t *reader, const char *filename) {
     int fd = open(filename, O_RDONLY, 0644);
     MP_THREAD_GIL_ENTER();
     if (fd < 0) {
-        mp_raise_OSError(errno);
+        mp_raise_OSError_with_filename(errno, filename);
     }
     mp_reader_new_file_from_fd(reader, fd, true);
 }