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/platform/emscripten/mod.rs

@@ -559,8 +559,8 @@ impl Window {
     }
 
     #[inline]
-    pub fn set_cursor_position(&self, _position: LogicalPosition) -> Result<(), ()> {
-        Err(())
+    pub fn set_cursor_position(&self, _position: LogicalPosition) -> Result<(), String> {
+        Err("Setting cursor position is not possible on Emscripten.".to_owned())
     }
 
     #[inline]