Improve error messages for extension script errors

Browsers won't tell us any specifics, but the console will let the user find out what happened.

src/extension-support/extension-worker.js

@@ -14,7 +14,9 @@ const loadScripts = url => {
         return new Promise((resolve, reject) => {
             const script = document.createElement('script');
             script.onload = () => resolve();
-            script.onerror = () => reject(new Error(`Error when loading custom extension script: ${url}`));
+            script.onerror = () => {
+                reject(new Error(`Error in sandboxed script: ${url}. Check the console for more information.`));
+            };
             script.src = url;
             document.body.appendChild(script);
         });

src/extension-support/tw-unsandboxed-extension-runner.js

@@ -49,7 +49,9 @@ const loadUnsandboxedExtension = (extensionURL, vm) => new Promise((resolve, rej
     createUnsandboxedExtensionAPI(vm).then(resolve);
 
     const script = document.createElement('script');
-    script.onerror = () => reject(new Error(`Error in unsandboxed script ${extensionURL}`));
+    script.onerror = () => {
+        reject(new Error(`Error in unsandboxed script ${extensionURL}. Check the console for more information.`));
+    };
     script.src = extensionURL;
     document.body.appendChild(script);
 }).then(objects => {