Handle FileNotFoundError in ipc.listen

Fixes #1530

CHANGELOG.asciidoc

@@ -87,6 +87,7 @@ Fixed
 - Fixed rebinding of keybindings being case-sensitive
 - Fix for tab indicators getting lost when moving tabs
 - Fixed handling of backspace in number hinting mode
+- Fixed `FileNotFoundError` when starting in some cases on old Qt versions
 
 v0.6.2
 ------

qutebrowser/misc/ipc.py

@@ -219,7 +219,13 @@ class IPCServer(QObject):
             # https://bugreports.qt.io/browse/QTBUG-48635
             #
             # This means we only use setSocketOption on Windows...
-            os.chmod(self._server.fullServerName(), 0o700)
+            try:
+                os.chmod(self._server.fullServerName(), 0o700)
+            except FileNotFoundError:
+                # https://github.com/The-Compiler/qutebrowser/issues/1530
+                # The server doesn't actually exist even if ok was reported as
+                # True, so report this as an error.
+                raise ListenError(self._server)
 
     @pyqtSlot('QLocalSocket::LocalSocketError')
     def on_error(self, err):