Simplified the reset of the _svg_warning_displayed flag in RichIPythonWidget

By overriding export_html(), and reseting the flag prior to the html
export.

This avoids repeated messages for a single document save, but repeats the
message for subsequent saves, including in new tabs and across kernel
restarts.

IPython/frontend/qt/console/rich_ipython_widget.py

@@ -29,6 +29,8 b' class RichIPythonWidget(IPythonWidget):'
     # RichIPythonWidget protected class variables.
     _payload_source_plot = 'IPython.zmq.pylab.backend_payload.add_plot_payload'
     _jpg_supported = Bool(False)
+    _svg_warning_displayed = False
+
     #---------------------------------------------------------------------------
     # 'object' interface
     #---------------------------------------------------------------------------
@@ -53,6 +55,19 b' class RichIPythonWidget(IPythonWidget):'
 
 
     #---------------------------------------------------------------------------
+    # 'ConsoleWidget' public interface overides
+    #---------------------------------------------------------------------------
+
+    def export_html(self):
+        """ Shows a dialog to export HTML/XML in various formats.
+
+        Overridden in order to reset the _svg_warning_displayed flag.
+        """
+        self._svg_warning_displayed = False
+        super(RichIPythonWidget, self).export_html()
+
+
+    #---------------------------------------------------------------------------
     # 'ConsoleWidget' protected interface
     #---------------------------------------------------------------------------
 
@@ -231,24 +246,13 b' class RichIPythonWidget(IPythonWidget):'
             try:
                 svg = str(self._name_to_svg_map[match.group("name")])
             except KeyError:
-
-                # Determine whether this is a new export by checking whether
-                # we've attempted to export this svg previously.  If so, then
-                # reset the flag that determines whether the warning has been
-                # displayed or not.
-                if match.group("name") in getattr(self, '_svgs_processed', set()):
-                    self._svg_warning_displayed = False
-
-                # Display this warning exactly once per export attempt.
-                if not getattr(self, '_svg_warning_displayed', False):
+                if not self._svg_warning_displayed:
                     QtGui.QMessageBox.warning(self, 'Error converting PNG to SVG.',
                         'Cannot convert a PNG to SVG.  To fix this, add this to your ipython config:\n\n'
                         '\tc.InlineBackendConfig.figure_format = \'svg\'\n\n'
                         'And regenerate the figures.',
                                               QtGui.QMessageBox.Ok)
                     self._svg_warning_displayed = True
-                    self._svgs_processed = set()
-                self._svgs_processed.add(match.group("name"))
                 return "<b>Cannot convert a PNG to SVG.  </b>To fix this, add this to your config: <span>c.InlineBackendConfig.figure_format = 'svg'</span> and regenerate the figures."
 
             # Not currently checking path, because it's tricky to find a