Avoid re defining cases for GIoErrorEnum when we already handle them
through GFileError, so remove code duplication and just rely on
g_file_error_from_errno() to compute the file error and then use
g_io_error_from_file_error() to get the possible IOError.

In case it's something not handled as GFileError, we can use the same
logic as before.

This is now a safe change to do as we have covered all the supported
cases in tests.