[breaking] render raw response when unexpected error occurs in endpoint

..instead of failing completely. Also catch possible fetch error (which would occur when network fails).
Closes #4801

There are still some code paths which would bubble up to the handler, but these are of the kind "you use a wrong SvelteKit option"

.changeset/few-ghosts-attend.md

@@ -0,0 +1,5 @@
+---
+'@sveltejs/kit': patch
+---
+
+[breaking] catch and render raw response when unexpected error occurs in endpoint

packages/kit/src/runtime/server/endpoint.js

@@ -60,7 +60,7 @@ export async function render_endpoint(event, mod, state) {
 				headers: { Location: error.location }
 			});
 		} else {
-			throw error;
+			return new Response(/** @type {Error} */ (error)?.message || 'Error', { status: 500 });
 		}
 	}
 }

packages/kit/src/runtime/server/index.js

@@ -315,7 +315,11 @@ export async function respond(request, options, state) {
 
 				// we can't load the endpoint from our own manifest,
 				// so we need to make an actual HTTP request
-				return await fetch(request);
+				try {
+					return await fetch(request);
+				} catch (e) {
+					return new Response(coalesce_to_error(e).message, { status: 500 });
+				}
 			},
 
 			// TODO remove for 1.0

packages/kit/src/runtime/server/page/index.js

@@ -330,7 +330,7 @@ export async function render_page(event, route, page, options, state, resolve_op
 		});
 	} catch (error) {
 		// if we end up here, it means the data loaded successfull
-		// but the page failed to render
+		// but the page failed to render, or that a prerendering error occurred
 		options.handle_error(/** @type {Error} */ (error), event);
 
 		return await respond_with_error({