WebAgg: raise WebAggApplication.started flag before blocking

Since Tornado's IOLoop start() blocks until the loop is closed,
cls.started used to be set to True *after* an application actually
stops working.
Revert running() check since it no longer exists in newer versions
of Tornado and will never return back.

Author: vmarkovtsev

lib/matplotlib/backends/backend_webagg.py

@@ -369,17 +369,24 @@ def start(cls):
         if cls.started:
             return
 
-        if not tornado.ioloop.IOLoop.instance().running():
-            print("Press Ctrl+C to stop server")
-            try:
-                tornado.ioloop.IOLoop.instance().start()
-            except KeyboardInterrupt:
-                print("Server stopped")
-        else:
-            print("Server is running in an existing Tornado IOLoop")
-
+        # Set the flag to True *before* blocking on IOLoop.instance().start()
         cls.started = True
 
+        """
+        IOLoop.running() was removed as of Tornado 2.4; see for example
+        https://groups.google.com/forum/#!topic/python-tornado/QLMzkpQBGOY
+        Thus there is no correct way to check if the loop has already been
+        launched. We may end up with two concurrently running loops in that
+        unlucky case with all the expected consequences.
+        """
+        print("Press Ctrl+C to stop server")
+        try:
+            tornado.ioloop.IOLoop.instance().start()
+        except KeyboardInterrupt:
+            print("Server stopped")
+        finally:
+            cls.started = False
+
 
 def ipython_inline_display(figure):
     import tornado.template