Fix Window platform support documentation

This resolves various problems with the documentation about platform
support on the Window struct.

It also completely removes pointless runtime errors in favor of
consistent no-ops on all platforms that do not support a certain
features.

src/platform_impl/android/mod.rs

@@ -419,7 +419,7 @@ impl Window {
     }
 
     pub fn set_inner_size(&self, _size: Size) {
-        panic!("Cannot set window size on Android");
+        warn!("Cannot set window size on Android");
     }
 
     pub fn outer_size(&self) -> PhysicalSize<u32> {
@@ -441,7 +441,7 @@ impl Window {
     pub fn set_maximized(&self, _maximized: bool) {}
 
     pub fn set_fullscreen(&self, _monitor: Option<window::Fullscreen>) {
-        panic!("Cannot set fullscreen on Android");
+        warn!("Cannot set fullscreen on Android");
     }
 
     pub fn fullscreen(&self) -> Option<window::Fullscreen> {

src/platform_impl/ios/window.rs

@@ -142,7 +142,7 @@ impl Inner {
     }
 
     pub fn set_inner_size(&self, _size: Size) {
-        unimplemented!("not clear what `Window::set_inner_size` means on iOS");
+        warn!("not clear what `Window::set_inner_size` means on iOS");
     }
 
     pub fn set_min_inner_size(&self, _dimensions: Option<Size>) {

src/platform_impl/web/window.rs

@@ -171,14 +171,12 @@ impl Window {
 
     #[inline]
     pub fn set_cursor_position(&self, _position: Position) -> Result<(), ExternalError> {
-        // Intentionally a no-op, as the web does not support setting cursor positions
-        Ok(())
+        Err(ExternalError::NotSupported(NotSupportedError::new()))
     }
 
     #[inline]
     pub fn set_cursor_grab(&self, _grab: bool) -> Result<(), ExternalError> {
-        // Intentionally a no-op, as the web does not (properly) support grabbing the cursor
-        Ok(())
+        Err(ExternalError::NotSupported(NotSupportedError::new()))
     }
 
     #[inline]

src/platform_impl/windows/window.rs

@@ -609,7 +609,7 @@ impl Window {
 
     #[inline]
     pub fn set_ime_position(&self, _position: Position) {
-        unimplemented!();
+        warn!("`Window::set_ime_position` is ignored on Windows")
     }
 
     #[inline]