Fix refreshBlocks error log

Stringification is more likely to produce a helpful output than
JSON.stringify on error objects.

Old message:
Failed to refresh built-in extension primitives: {}
New message:
Failed to refresh built-in extension primitives: TypeError: Cannot destructure property 'provider' of ...

src/extension-support/extension-manager.js

@@ -280,7 +280,7 @@ class ExtensionManager {
                     dispatch.call('runtime', '_refreshExtensionPrimitives', info);
                 })
                 .catch(e => {
-                    log.error(`Failed to refresh built-in extension primitives: ${JSON.stringify(e)}`);
+                    log.error(`Failed to refresh built-in extension primitives: ${e}`);
                 })
         );
         return Promise.all(allPromises);