Change set_cursor_position to return Result<(), String> (#562)

* Change set_cursor_position to return Result<(), String>

This is now consistent with `grab_cursor`, and
enables `window.set_cursor_position(x, y)?` in functions
that return `Result<_, Box<Error>>`.

* Adjust error handling of unimplemented cusor opertions in wayland

* The final nitpick

* Actually one more

src/window.rs

@@ -322,7 +322,7 @@ impl Window {
 
     /// Changes the position of the cursor in window coordinates.
     #[inline]
-    pub fn set_cursor_position(&self, position: LogicalPosition) -> Result<(), ()> {
+    pub fn set_cursor_position(&self, position: LogicalPosition) -> Result<(), String> {
         self.window.set_cursor_position(position)
     }