Fail during `backupMove()` doesn't throw exception

[adam-paterson]

rename() can fail fail on line GetCommand.php:229 and there is no error handling for this method.

[punkstar]

Need to check that the destination is writable before anything starts too.

[bobbyshaw]

It currently checks the directory is writable but we should update that check to be specifically checking the file path is writable and that we catch the error 👍