fix: handle middleware loading error

.changeset/selfish-rings-occur.md

@@ -0,0 +1,5 @@
+---
+'astro': patch
+---
+
+Correctly handle the error in case the middleware throws a runtime error

packages/astro/src/core/errors/errors-data.ts

@@ -780,6 +780,26 @@ export const LocalsNotAnObject = {
 	hint: 'If you tried to remove some information from the `locals` object, try to use `delete` or set the property to `undefined`.',
 } satisfies ErrorData;
 
+/**
+ * @docs
+ * @description
+ * Thrown in development mode when middleware throws an error while attempting to loading it.
+ *
+ * For example:
+ * ```ts
+ * import {defineMiddleware} from "astro:middleware";
+ * throw new Error("Error thrown while loading the middleware.")
+ * export const onRequest = defineMiddleware(() => {
+ *   return "string"
+ * });
+ * ```
+ */
+export const MiddlewareCantBeLoaded = {
+	name: 'MiddlewareCantBeLoaded',
+	title: "Can't load the middleware.",
+	message: 'The middleware thrown an error while Astro was trying to loading it.',
+} satisfies ErrorData;
+
 /**
  * @docs
  * @see

packages/astro/src/core/middleware/loadMiddleware.ts

@@ -1,5 +1,7 @@
 import type { ModuleLoader } from '../module-loader/index.js';
 import { MIDDLEWARE_MODULE_ID } from './vite-plugin.js';
+import { MiddlewareCantBeLoaded } from '../errors/errors-data.js';
+import { AstroError } from '../errors/index.js';
 
 /**
  * It accepts a module loader and the astro settings, and it attempts to load the middlewares defined in the configuration.
@@ -8,9 +10,9 @@ import { MIDDLEWARE_MODULE_ID } from './vite-plugin.js';
  */
 export async function loadMiddleware(moduleLoader: ModuleLoader) {
 	try {
-		const module = await moduleLoader.import(MIDDLEWARE_MODULE_ID);
-		return module;
-	} catch {
-		return void 0;
+		return await moduleLoader.import(MIDDLEWARE_MODULE_ID);
+	} catch (error: any) {
+		const astroError = new AstroError(MiddlewareCantBeLoaded, undefined, { cause: error });
+		throw astroError;
 	}
 }