improve graphql error handling

web/src/app/[...url]/page.tsx

@@ -51,12 +51,14 @@ export async function generateStaticParams() {
     }
   `);
   const { data, error } = await getClient().query(allGenericSlugsQuery, {});
-
-  if (data === undefined || error) {
-    throw new Error("failed to generate static params");
+  if (error) {
+    throw new Error(error.message);
+  }
+  if (!data?.pages) {
+    throw new Error("Failed to generate static params for generic subpages");
   }
 
-  return data?.pages.map((page: any) => {
+  return data.pages.map((page: any) => {
     // wagtail-grapple prepends the home page slug to the full path on multisite setups
     // we also strip the trailing slash
     const urlPath: string[] = page.urlPath
@@ -79,12 +81,14 @@ export async function generateMetadata(
     urlPath: urlPath,
   });
 
-  const page = (data?.page ?? []) as GenericFragment;
-
-  if (!page) {
+  if (error) {
+    throw new Error(error.message);
+  }
+  if (!data?.page) {
     return null;
   }
 
+  const page = data.page as GenericFragment;
   const metadata = await getSeoMetadata(page, parent);
   return metadata;
 }
@@ -95,13 +99,16 @@ export default async function Page({ params }: { params: { url: string[] } }) {
   const { data, error } = await getClient().query(genericPageByUrlPathQuery, {
     urlPath: urlPath,
   });
+  if (error) {
+    throw new Error(error.message);
+  }
 
-  const page = (data?.page ?? []) as GenericFragment;
-
-  if (!page) {
+  if (!data?.page) {
     return notFound();
   }
 
+  const page = data?.page as GenericFragment;
+
   return (
     <>
       <main className="site-main" id="main">