Add failure message for use of prerender.force

This commit can eventually be reverted, once the team thinks enough
people have updated and are using the new `prerender.onError` option. It
is purely here for ease of transition from `force` to `onError`.

packages/kit/src/core/config/index.spec.js

@@ -50,6 +50,8 @@ test('fills in defaults', () => {
 			prerender: {
 				crawl: true,
 				enabled: true,
+				// This can be removed entirely after we remove the validation error associated with using `force`
+				force: undefined,
 				onError: 'fail',
 				pages: ['*']
 			},
@@ -150,6 +152,8 @@ test('fills in partial blanks', () => {
 			prerender: {
 				crawl: true,
 				enabled: true,
+				// This can be removed entirely after we remove the validation error associated with using `force`
+				force: undefined,
 				onError: 'fail',
 				pages: ['*']
 			},

packages/kit/src/core/config/options.js

@@ -121,6 +121,21 @@ const options = {
 				children: {
 					crawl: expect_boolean(true),
 					enabled: expect_boolean(true),
+					force: {
+						type: 'leaf',
+						default: undefined,
+						validate: (option, keypath) => {
+							if (typeof option !== undefined) {
+								const newSetting = option ? 'continue' : 'fail';
+								const needsSetting = newSetting === 'continue';
+								throw new Error(
+									`${keypath} has been removed in favor of \`onError\`. In your case, set \`onError\` to "${newSetting}"${
+										needsSetting ? '' : ' (or leave it undefined)'
+									} to get the same behavior as you would with \`force: ${JSON.stringify(option)}\``
+								);
+							}
+						}
+					},
 					onError: {
 						type: 'leaf',
 						default: 'fail',

packages/kit/src/core/config/test/index.js

@@ -54,7 +54,7 @@ async function testLoadDefaultConfig(path) {
 				exclude: []
 			},
 			paths: { base: '', assets: '/.' },
-			prerender: { crawl: true, enabled: true, onError: 'fail', pages: ['*'] },
+			prerender: { crawl: true, enabled: true, force: undefined, onError: 'fail', pages: ['*'] },
 			router: true,
 			ssr: true,
 			target: null,