os: return PathError on RemoveAll with trailing dots

Return a PathError instead of an unwrapped syscall.EINVAL if the path
ends with dots.

As suggested by Roger Peppe in CL 150158.

Change-Id: I4d82a6ff64a979b67a843a1cc4fea58ed9326aed
Reviewed-on: https://go-review.googlesource.com/c/150160
Run-TryBot: Tobias Klauser <tobias.klauser@gmail.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Ian Lance Taylor <iant@golang.org>

src/os/removeall_at.go

@@ -22,7 +22,7 @@ func RemoveAll(path string) error {
 	// The rmdir system call does not permit removing ".",
 	// so we don't permit it either.
 	if endsWithDot(path) {
-		return syscall.EINVAL
+		return &PathError{"RemoveAll", path, syscall.EINVAL}
 	}
 
 	// RemoveAll recurses by deleting the path base from