[wesnothd] better error handling for mp_mod_group setting

[soliton-]

When the [user_handler] mp_mod_group setting is empty or doesn't contain a valid number wesnothd just terminates with:

error server: terminated by C++ exception: stoi

Would be nice if that would produce a more descriptive error message. Terminating is still okay unless the key was not in the config at all. I'd say if the key is not there, empty or 0 that should deactivate the group check.