settings: propagate certificate blob write failure in keyfile writer

When nm_utils_file_set_contents() fails to write a certificate blob to
disk, cert_writer() only logged a warning and silently continued. The
overall write operation would "succeed", producing a keyfile missing the
certificate entry entirely. This could lead to a broken 802.1X
connection profile that fails authentication at runtime.

Propagate the error via g_set_error() so that nm_keyfile_write() returns
failure and the profile write is properly aborted.

Fixes: ecca85066d ('keyfile: write certificates and keys in a human-readable manner')

src/core/settings/plugins/keyfile/nms-keyfile-writer.c

@@ -143,7 +143,9 @@ cert_writer(NMConnection                     *connection,
                                             vtable->setting_key,
                                             strrchr(new_path, '/') + 1);
         } else {
-            nm_log_warn(LOGD_SETTINGS,
+            g_set_error(error,
+                        NM_SETTINGS_ERROR,
+                        NM_SETTINGS_ERROR_FAILED,
                         "keyfile: %s.%s: failed to write certificate to file %s: %s",
                         setting_name,
                         vtable->setting_key,