Fix when 'remove' throws

The precedence was wrong on checking whether `->remove` succeeds, meaning that it never threw an exception for actual errors, but did throw one when trying to delete a file called '0' (and not delete it).

A couple of failing tests to demonstrate the problems, plus a fix.

Also, a tweak to the docs for `remove`, which previously made it sound like the difference from Perl's built-in `unlink` command is that it throws less, but `unlink` by default (without `autodie`) doesn't throw at all. Rewritten in a way which doesn't imply anything when `unlink` throws, so is correct for both default and `autodie` users.