Add basic port validation (#679)

README.md

@@ -30,8 +30,9 @@ int main() {
         res->end(const char *, size_t);
     });
 
-    h.listen(3000);
-    h.run();
+    if (h.listen(3000)) {
+        h.run();
+    }
 }
 ```
 Get the sources of the uws.chat server [here](https://github.com/uWebSockets/website/blob/master/main.cpp). Learn from the tests [here](tests/main.cpp).

examples/echo.cpp

@@ -8,6 +8,7 @@ int main()
         ws->send(message, length, opCode);
     });
 
-    h.listen(3000);
-    h.run();
+    if (h.listen(3000)) {
+        h.run();
+    }
 }