fix(block): Add missing error pin to Exa Contents Block (#9604)

<!-- Clearly explain the need for these changes: -->

### Changes 🏗️

The Exa Block has a missing error pin which prevents error handling,
this adds that.

autogpt_platform/backend/backend/blocks/exa/contents.py

@@ -51,6 +51,7 @@ class ExaContentsBlock(Block):
             description="List of document contents",
             default=[],
         )
+        error: str = SchemaField(description="Error message if the request failed")
 
     def __init__(self):
         super().__init__(