added try block to backend_gtk3.py

Added try block around `draw` in `idle_draw`.  If `draw`
raises an exception the figure is never marked idle again and
pyplot functions will not work as expect.

Author: tacaswell

lib/matplotlib/backends/backend_gtk3.py

@@ -322,8 +322,10 @@ def draw(self):
 
     def draw_idle(self):
         def idle_draw(*args):
-            self.draw()
-            self._idle_draw_id = 0
+            try:
+                self.draw()
+            finally:
+                self._idle_draw_id = 0
             return False
         if self._idle_draw_id == 0:
             self._idle_draw_id = GObject.idle_add(idle_draw)