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