* Use NIO in tryOnceDeleteFile and makeWritable

* Don't try to set PosixFileAttributes on Windows

* Do not create arbitrary exceptions in makeWritable to fix test failures on Windows

* Remove unhelpful layer of exception wrapping

* Add test exercising Util#makeWritable in Util#tryOnceDeleteFile

* Add test for deleting a non-existant file

* Return early if changing permissions with NIO succeeds