Gets a GFileError constant based on the passed-in errno.
For example, if you pass in EEXIST this function returns G_FILE_ERROR_EXIST. Unlike errno values, you can portably assume that all GFileError values will exist.
Normally a GFileError value goes into a GError returned from a function that manipulates files. So you would use g_file_error_from_errno() when constructing a GError.
| err_no | an "errno" value |
| GFileError corresponding to the given errno |