Show error on UI when API save fails

[malberts]

When updating Rules via the UI, there should be an error message if the API call fails:

Rules/modules/ext.rules/composables/useRules.js

Lines 69 to 81 in aa5f7f0

	async function saveRules( api, title ) {
	saving.value = true;
	try {
	const content = JSON.stringify( { rules: rules.value } );
	return await api.postWithToken( 'csrf', {
	action: 'edit',
	title,
	text: content
	} );
	} finally {
	saving.value = false;
	}
	}

This can happen for various reasons, but as an example, if the user is logged out while editing:

Screencast_20250730_021148.webm