Catch `FileNotFound` when clearing invalid sessions

The server now consumes the FNF error if an invalid session is found but
is deleted in an earlier thread.

Fixes #577

app/routes.py

@@ -85,7 +85,11 @@ def session_required(f):
                 pass
 
         for invalid_session in invalid_sessions:
+            try:
                 os.remove(invalid_session)
+            except FileNotFoundError:
+                # Don't throw error if the invalid session has been removed
+                pass
 
         return f(*args, **kwargs)