fix: Fail early when neither user nor system is requested

The module can be configured to neither request the user nor the system
session.

In fact does this display non-valuable information to the user and can
be considered as abuse case of the configuration.

Therefore throw an exception when neither user nor system session are
requested.

Signed-off-by: Steffen Kothe <steffen.kothe@skothe.net>

src/modules/systemd_failed_units.cpp

@@ -47,6 +47,9 @@ SystemdFailedUnits::SystemdFailedUnits(const std::string& id, const Json::Value&
     user_proxy->signal_signal().connect(sigc::mem_fun(*this, &SystemdFailedUnits::notify_cb));
   }
 
+  if (!user_proxy && !system_proxy)
+    throw std::runtime_error("Neither system nor user status is requested.");
+
   updateData();
   /* Always update for the first time. */
   dp.emit();