x11: use more information in X11 "not supported" errors

This makes it so, when X11 fails to initialize due to not loading a
library, it provides more verbose information on what exactly happened.

Fixes rust-windowing#3883.
Signed-off-by: John Nunley <[email protected]>
Co-authored-by: Kirill Chibisov <[email protected]>

src/platform_impl/linux/mod.rs

@@ -117,6 +117,8 @@ pub(crate) static X11_BACKEND: Lazy<Mutex<Result<Arc<XConnection>, XNotSupported
 pub enum OsError {
     Misc(&'static str),
     #[cfg(x11_platform)]
+    XNotSupported(XNotSupported),
+    #[cfg(x11_platform)]
     XError(Arc<X11Error>),
     #[cfg(wayland_platform)]
     WaylandError(Arc<wayland::WaylandError>),
@@ -127,6 +129,8 @@ impl fmt::Display for OsError {
         match *self {
             OsError::Misc(e) => _f.pad(e),
             #[cfg(x11_platform)]
+            OsError::XNotSupported(ref e) => fmt::Display::fmt(e, _f),
+            #[cfg(x11_platform)]
             OsError::XError(ref e) => fmt::Display::fmt(e, _f),
             #[cfg(wayland_platform)]
             OsError::WaylandError(ref e) => fmt::Display::fmt(e, _f),
@@ -761,7 +765,9 @@ impl<T: 'static> EventLoop<T> {
     fn new_x11_any_thread() -> Result<EventLoop<T>, EventLoopError> {
         let xconn = match X11_BACKEND.lock().unwrap().as_ref() {
             Ok(xconn) => xconn.clone(),
-            Err(_) => return Err(EventLoopError::NotSupported(NotSupportedError::new())),
+            Err(err) => {
+                return Err(EventLoopError::Os(os_error!(OsError::XNotSupported(err.clone()))))
+            },
         };
 
         Ok(EventLoop::X(x11::EventLoop::new(xconn)))