Add error message for Wepoll (#54)

* Add error message for Wepoll

* Avoid unsupported error kind where it is not supported

build.rs

@@ -18,4 +18,8 @@ fn main() {
     if !cfg.probe_rustc_version(1, 63) {
         autocfg::emit("polling_no_io_safety");
     }
+
+    if !cfg.probe_rustc_version(1, 53) {
+        autocfg::emit("polling_no_unsupported_error_kind");
+    }
 }

src/wepoll.rs

@@ -42,7 +42,16 @@ impl Poller {
     pub fn new() -> io::Result<Poller> {
         let handle = unsafe { we::epoll_create1(0) };
         if handle.is_null() {
-            return Err(io::Error::last_os_error());
+            return Err(io::Error::new(
+                #[cfg(not(polling_no_unsupported_error_kind))]
+                io::ErrorKind::Unsupported,
+                #[cfg(polling_no_unsupported_error_kind)]
+                io::ErrorKind::Other,
+                format!(
+                    "Failed to initialize Wepoll: {}\nThis usually only happens for old Windows or Wine.",
+                    io::Error::last_os_error()
+                )
+            ));
         }
         let notified = AtomicBool::new(false);
         log::trace!("new: handle={:?}", handle);