Don't use 0 as window ID for early messages.

The issue here was that only message calls with 'current' as window get cached
if the window is unavailable. We used 0 instead.

Fixes #815.
See #812.

qutebrowser/app.py

@@ -272,8 +272,8 @@ def process_pos_args(args, via_ipc=False, cwd=None):
             try:
                 url = urlutils.fuzzy_url(cmd, cwd, relative=True)
             except urlutils.FuzzyUrlError as e:
-                message.error(0, "Error in startup argument '{}': {}".format(
-                    cmd, e))
+                message.error('current', "Error in startup argument '{}': "
+                              "{}".format(cmd, e))
             else:
                 open_target = config.get('general', 'new-instance-open-target')
                 background = open_target in ('tab-bg', 'tab-bg-silent')
@@ -302,8 +302,8 @@ def _open_startpage(win_id=None):
                 try:
                     url = urlutils.fuzzy_url(urlstr, do_search=False)
                 except urlutils.FuzzyUrlError as e:
-                    message.error(0, "Error when opening startpage: {}".format(
-                        e))
+                    message.error('current', "Error when opening startpage: "
+                                  "{}".format(e))
                     tabbed_browser.tabopen(QUrl('about:blank'))
                 else:
                     tabbed_browser.tabopen(url)

qutebrowser/browser/quickmarks.py

@@ -75,7 +75,8 @@ class QuickmarkManager(QObject):
                 try:
                     key, url = line.rsplit(maxsplit=1)
                 except ValueError:
-                    message.error(0, "Invalid quickmark '{}'".format(line))
+                    message.error('current', "Invalid quickmark '{}'".format(
+                        line))
                 else:
                     self.marks[key] = url
             filename = os.path.join(standarddir.config(), 'quickmarks')