Print a slightly clearer message when failing to spawn a thread

src/libstd/thread/mod.rs

@@ -607,7 +607,7 @@ impl Builder {
 pub fn spawn<F, T>(f: F) -> JoinHandle<T> where
     F: FnOnce() -> T, F: Send + 'static, T: Send + 'static
 {
-    Builder::new().spawn(f).unwrap()
+    Builder::new().spawn(f).expect("failed to spawn thread")
 }
 
 /// Gets a handle to the thread that invokes it.