Handle SessionError when shutting down sessions

Fixes #810

doc/changelog.asciidoc

@@ -56,6 +56,8 @@ Fixed
   mode and get hints assigned correctly.
 - When checkmarks, radio buttons and some other elements are styled via the
   Bootstrap CSS framework, they now get hints correctly.
+- When the session file isn't writable when qutebrowser is quit, an error is
+  now logged instead of crashing.
 
 v1.13.1 (unreleased)
 --------------------

qutebrowser/misc/sessions.py

@@ -85,12 +85,15 @@ def shutdown(session: typing.Optional[ArgType], last_window: bool) -> None:
     if session_manager is None:
         return
 
-    if session is not None:
-        session_manager.save(session, last_window=last_window,
-                             load_next_time=True)
-    elif config.val.auto_save.session:
-        session_manager.save(default, last_window=last_window,
-                             load_next_time=True)
+    try:
+        if session is not None:
+            session_manager.save(session, last_window=last_window,
+                                load_next_time=True)
+        elif config.val.auto_save.session:
+            session_manager.save(default, last_window=last_window,
+                                load_next_time=True)
+    except SessionError as e:
+        log.sessions.error("Failed to save session: {}".format(e))
 
     session_manager.delete_autosave()