downloads: don't crash on OSError in open-download

Fixes the crash in #1725, but does not provide a solution. The browser
won't crash, but the file won't be downloaded and opened either.

qutebrowser/browser/webkit/downloads.py

@@ -947,7 +947,13 @@ class DownloadManager(QAbstractListModel):
             download.set_filename(target.filename)
         elif isinstance(target, usertypes.OpenFileDownloadTarget):
             tmp_manager = objreg.get('temporary-downloads')
+            try:
                 fobj = tmp_manager.get_tmpfile(suggested_filename)
+            except OSError as exc:
+                msg = "Download error: {}".format(exc)
+                message.error(self._win_id, msg)
+                download.cancel()
+                return
             download.finished.connect(download.open_file)
             download.autoclose = True
             download.set_fileobj(fobj)