Add error messages for QtWebEngine downloads

Fixes #2164

CHANGELOG.asciidoc

@@ -70,6 +70,8 @@ Changed
  closes the current tab.
- (QtWebEngine) With Qt 5.9, `content -> cookies-store` can now be set without
  a restart.
- (QtWebEngine) With Qt 5.9, better error messages are now shown for failed
  downloads.
- The adblocker now also blocks non-GET requests (e.g. POST).
- `javascript:` links can now be hinted.
- `:view-source`, `:tab-clone` and `:navigate --tab` now don't open the tab as

qutebrowser/browser/webengine/webenginedownloads.py

@@ -77,7 +77,12 @@ def _on_state_changed(self, state):
        elif state == QWebEngineDownloadItem.DownloadInterrupted:
            self.successful = False
            # https://bugreports.qt.io/browse/QTBUG-56839
            self._die("Download failed")
            try:
                reason = self._qt_item.interruptReasonString()
            except AttributeError:
                # Qt < 5.9
                reason = "Download failed"
            self._die(reason)
        else:
            raise ValueError("_on_state_changed was called with unknown state "
                             "{}".format(state_name))