fix: graciouslyFetch returns undefined on network error, crashes updates_for callers

When a fetch fails (network error, CORS, timeout), graciouslyFetch()
catches the error but implicitly returns undefined. Callers in
updates_for_element.js then crash with TypeError when calling .text()
on the undefined result.

Fix: return new Response('') in the catch block so callers always
receive a valid Response object. Also add defensive guards at both
call sites for defense in depth.

Fixes #310

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

Author: JeremiahChurch

javascript/elements/updates_for_element.js

@@ -120,7 +120,7 @@ export default class UpdatesForElement extends SubscribingElement {
           const response = await graciouslyFetch(url, {
             'X-Cable-Ready': 'update'
           })
-          this.html[url] = await response.text()
+          if (response) this.html[url] = await response.text()
         }
       })
     )
@@ -254,7 +254,7 @@ class Block {
           )
 
           const frameTemplate = document.createElement('template')
-          frameTemplate.innerHTML = await frameResponse.text()
+          if (frameResponse) frameTemplate.innerHTML = await frameResponse.text()
 
           // recurse here to get all nested eager loaded frames
           await this.resolveTurboFrames(frameTemplate.content)

javascript/utils.js

@@ -202,6 +202,7 @@ async function graciouslyFetch (url, additionalHeaders) {
     return response
   } catch (e) {
     console.error(`Could not fetch ${url}`)
+    return new Response('')
   }
 }