systemadm: report GLib.Error only to stderr

When a GLib.Error happens, it is likely that showing a message box would fail
too.

https://bugzilla.redhat.com/show_bug.cgi?id=716663

src/systemadm.vala

@@ -1011,7 +1011,7 @@ int main(string[] args) {
         } catch (IOError e) {
                 show_error(e.message);
         } catch (GLib.Error e) {
-                show_error(e.message);
+                stderr.printf("%s\n", e.message);
         }
 
         return 0;