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))