In PLAIN_TEXT_FILE, we have `is_sequence_an_expected_numeric' which is currently not exported. If we were to export it, we could use it for letting the user know about the failure to read.

However for the real/double variant, it is harder and requires that we put a rescue clause in `read_double' and `read_real' to catch failures, so I'll need to modify those externals so that we get a better idea of what the failure is.