ifcfg-rh: add helper to set unsupported error

The ifcfg-rh plugin is now deprecated and in bugfixes-only mode. When
users try to set a property that is not supported by the plugin, we
need to report an error.

Add an helper function to set such error. Also, introduce a new error
code so that the situation can be detected and dealt with
programmatically.

src/core/settings/plugins/ifcfg-rh/nms-ifcfg-rh-writer.c

@@ -61,6 +61,24 @@
 
 /*****************************************************************************/
 
+static void _nm_unused
+set_error_unsupported(GError      **error,
+                      NMConnection *connection,
+                      const char   *name,
+                      gboolean      is_setting)
+{
+    g_set_error(error,
+                NM_SETTINGS_ERROR,
+                NM_SETTINGS_ERROR_NOT_SUPPORTED_BY_PLUGIN,
+                "The ifcfg-rh plugin doesn't support %s '%s'. If you are modifying an existing "
+                "connection profile saved in ifcfg-rh format, please migrate the connection to "
+                "keyfile using 'nmcli connection migrate %s' or via the Update2() D-Bus API "
+                "and try again.",
+                is_setting ? "setting" : "property",
+                name,
+                nm_connection_get_uuid(connection));
+};
+
 static void
 save_secret_flags(shvarFile *ifcfg, const char *key, NMSettingSecretFlags flags)
 {

src/libnm-core-public/nm-errors.h

@@ -253,6 +253,9 @@ GQuark nm_secret_agent_error_quark(void);
  * @NM_SETTINGS_ERROR_VERSION_ID_MISMATCH: The profile's VersionId mismatched
  *   and the update is rejected. See the "version-id" argument to Update2()
  *   method. Since 1.44.
+ * @NM_SETTINGS_ERROR_NOT_SUPPORTED_BY_PLUGIN: the requested operation is not
+ *   supported by the settings plugin currently in use for the specified object.
+ *   Since: 1.44.
  *
  * Errors related to the settings/persistent configuration interface of
  * NetworkManager.
@@ -271,6 +274,7 @@ typedef enum {
     NM_SETTINGS_ERROR_INVALID_HOSTNAME,        /*< nick=InvalidHostname >*/
     NM_SETTINGS_ERROR_INVALID_ARGUMENTS,       /*< nick=InvalidArguments >*/
     NM_SETTINGS_ERROR_VERSION_ID_MISMATCH,     /*< nick=VersionIdMismatch >*/
+    NM_SETTINGS_ERROR_NOT_SUPPORTED_BY_PLUGIN, /*< nick=NotSupportedByPlugin >*/
 } NMSettingsError;
 
 GQuark nm_settings_error_quark(void);