feat: improve error message for handleHttpError and `handleMissingI…

…d` (#9621)

* patch: Improve error message for `handleHttpError` and `handleMissingId`

* chore: changest

* Update .changeset/tall-bags-sparkle.md

* put the message in the fallback handler

* oops

---------

Co-authored-by: Simon H <5968653+dummdidumm@users.noreply.github.com>
Co-authored-by: Rich Harris <git@rich-harris.dev>

.changeset/tall-bags-sparkle.md

@@ -0,0 +1,5 @@
+---
+'@sveltejs/kit': patch
+---
+
+fix: better error messages for handleable prerender failures

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

@@ -105,8 +105,6 @@ const get_defaults = (prefix = '') => ({
 			concurrency: 1,
 			crawl: true,
 			entries: ['*'],
-			handleHttpError: 'fail',
-			handleMissingId: 'fail',
 			origin: 'http://sveltekit-prerender'
 		},
 		version: {

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

@@ -201,17 +201,33 @@ const options = object(
 					return input;
 				}),
 
-				handleHttpError: validate('fail', (input, keypath) => {
-					if (typeof input === 'function') return input;
-					if (['fail', 'warn', 'ignore'].includes(input)) return input;
-					throw new Error(`${keypath} should be "fail", "warn", "ignore" or a custom function`);
-				}),
+				handleHttpError: validate(
+					(/** @type {any} */ { message }) => {
+						throw new Error(
+							message +
+								`\nTo suppress or handle this error, implement \`handleHttpError\` in https://kit.svelte.dev/docs/configuration#prerender`
+						);
+					},
+					(input, keypath) => {
+						if (typeof input === 'function') return input;
+						if (['fail', 'warn', 'ignore'].includes(input)) return input;
+						throw new Error(`${keypath} should be "fail", "warn", "ignore" or a custom function`);
+					}
+				),
 
-				handleMissingId: validate('fail', (input, keypath) => {
-					if (typeof input === 'function') return input;
-					if (['fail', 'warn', 'ignore'].includes(input)) return input;
-					throw new Error(`${keypath} should be "fail", "warn", "ignore" or a custom function`);
-				}),
+				handleMissingId: validate(
+					(/** @type {any} */ { message }) => {
+						throw new Error(
+							message +
+								`\nTo suppress or handle this error, implement \`handleMissingId\` in https://kit.svelte.dev/docs/configuration#prerender`
+						);
+					},
+					(input, keypath) => {
+						if (typeof input === 'function') return input;
+						if (['fail', 'warn', 'ignore'].includes(input)) return input;
+						throw new Error(`${keypath} should be "fail", "warn", "ignore" or a custom function`);
+					}
+				),
 
 				origin: validate('http://sveltekit-prerender', (input, keypath) => {
 					assert_string(input, keypath);