core/device: demote "strictly unamanged" error

The "device ... not available because device is strictly unmanaged" is
almost certainly the least interesting of the reasons why connection
can't be activated on a device.

Invent a new error level for it and demote it.

Before:

  Error: Connection activation failed: No suitable device found
         for this connection (device lo not available because
         device is strictly unmanaged).

After

  Error: Connection activation failed: No suitable device found
         for this connection (device eth0 not available because
         profile is not compatible with device (...)).

https://gitlab.freedesktop.org/NetworkManager/NetworkManager/-/merge_requests/1433

src/core/devices/nm-device.c

@@ -14851,7 +14851,7 @@ _nm_device_check_connection_available(NMDevice                      *self,
             if (!nm_device_get_managed(self, TRUE)) {
                 /* device is strictly unmanaged by authoritative unmanaged reasons. */
                 nm_utils_error_set_literal(error,
-                                           NM_UTILS_ERROR_CONNECTION_AVAILABLE_UNMANAGED_DEVICE,
+                                           NM_UTILS_ERROR_CONNECTION_AVAILABLE_STRICTLY_UNMANAGED_DEVICE,
                                            "device is strictly unmanaged");
                 return FALSE;
             }

src/libnm-glib-aux/nm-shared-utils.h

@@ -975,6 +975,7 @@ typedef enum {
      * message from the device that returned the *highest* error code,
      * in the hope that this message makes the most sense for the caller.
      * */
+    NM_UTILS_ERROR_CONNECTION_AVAILABLE_STRICTLY_UNMANAGED_DEVICE,
     NM_UTILS_ERROR_CONNECTION_AVAILABLE_INCOMPATIBLE,
     NM_UTILS_ERROR_CONNECTION_AVAILABLE_UNMANAGED_DEVICE,
     NM_UTILS_ERROR_CONNECTION_AVAILABLE_TEMPORARY,