Gets a GFileError constant based on the passed-in err_no.
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