libnmc-base: report explicit error if not gateway configured for openconnect

Rather than letting openconnect run, and whine that there's no gateway,
and making the user scroll up past the openconnect usage information,
give them an explicit error.

src/libnmc-base/nm-vpn-helpers.c

@@ -261,8 +261,16 @@ nm_vpn_openconnect_authenticate_helper(NMSettingVpn *s_vpn, GPtrArray *secrets,
     int         i, oc_argc = 0;
 
     /* Get gateway and port */
-    gw   = nm_setting_vpn_get_data_item(s_vpn, "gateway");
-    port = gw ? strrchr(gw, ':') : NULL;
+    gw = nm_setting_vpn_get_data_item(s_vpn, "gateway");
+    if (!gw) {
+        g_set_error(error,
+                    NM_VPN_PLUGIN_ERROR,
+                    NM_VPN_PLUGIN_ERROR_FAILED,
+                    _("no gateway configured"));
+        return FALSE;
+    }
+
+    port = strrchr(gw, ':');
 
     path = nm_utils_file_search_in_paths("openconnect",
                                          "/usr/sbin/openconnect",