Handle cancelled SSL prompts correctly

When the user pressed esc during an SSL prompt, the message.ask call
returned None, which was handled fine by QtWebKit (which simply used an
'if') but failed with QtWebEngine (which returned the value to Qt).

Fixes #2123.

qutebrowser/browser/shared.py

@@ -146,9 +146,13 @@ def ignore_certificate_errors(url, errors, abort_on):
         """.strip())
         msg = err_template.render(url=url, errors=errors)
 
-        return message.ask(title="Certificate errors - continue?", text=msg,
-                           mode=usertypes.PromptMode.yesno, default=False,
-                           abort_on=abort_on)
+        ignore = message.ask(title="Certificate errors - continue?", text=msg,
+                             mode=usertypes.PromptMode.yesno, default=False,
+                             abort_on=abort_on)
+        if ignore is None:
+            # prompt aborted
+            ignore = False
+        return ignore
     elif ssl_strict is False:
         log.webview.debug("ssl-strict is False, only warning about errors")
         for err in errors:

tests/end2end/features/prompts.feature

@@ -197,6 +197,14 @@ Feature: Prompts
         And I run :prompt-accept no
         Then a SSL error page should be shown
 
+    Scenario: SSL error with ssl-strict = ask -> abort
+        When I clear SSL errors
+        And I set network -> ssl-strict to ask
+        And I load an SSL page
+        And I wait for a prompt
+        And I run :leave-mode
+        Then a SSL error page should be shown
+
     # Geolocation
 
     Scenario: Always rejecting geolocation