stop retries after wrong content-type

It does not make sense to re-request the same resource with the same
parameters and expect a different content-type to be returned. Also
this makes the error message more sensible and understandable.

packages/backend/src/remote/activitypub/request.ts

@@ -121,9 +121,10 @@ export async function signedGet(_url: string, user: { id: User['id'] }): Promise
 				return false;
 			};
 
-			if (isActivitypub(res.headers.get('Content-Type'))) {
-				return await res.json();
+			if (!isActivitypub(res.headers.get('Content-Type'))) {
+				throw new Error('invalid response content type');
 			}
+			return await res.json();
 		}
 	}