Prevent questions from being aborted twice

When we abort a question (e.g. by closing a tab with SSL errors) while
another prompt is queued, the right prompt gets closed by the question
being aborted, but that in turn aborts the question again, affecting the
next prompt.

qutebrowser/utils/usertypes.py

@@ -407,6 +407,9 @@ class Question(QObject):
     @pyqtSlot()
     def abort(self):
         """Abort the question."""
+        if self.is_aborted:
+            log.misc.debug("Question was already aborted")
+            return
         self.is_aborted = True
         try:
             self.aborted.emit()

tests/end2end/features/prompts.feature

@@ -452,3 +452,18 @@ Feature: Prompts
               "authenticated": true,
               "user": "user2"
             }
+
+    # https://github.com/The-Compiler/qutebrowser/issues/1249#issuecomment-175205531
+    # https://github.com/The-Compiler/qutebrowser/pull/2054#issuecomment-258285544
+    Scenario: Interrupting SSL prompt during a notification prompt
+        When I set content -> notifications to ask
+        And I set network -> ssl-strict to ask
+        And I open data/prompt/notifications.html in a new tab
+        And I run :click-element id button
+        And I wait for a prompt
+        And I open about:blank in a new tab
+        And I load an SSL page
+        And I wait for a prompt
+        And I run :tab-close
+        And I run :prompt-accept yes
+        Then the javascript message "notification permission granted" should be logged