feat: enhance OpenAPI spec to include Error types (#1320)

# What does this PR do?

An API spec must talk about Error handling. This was a pretty glaring
omission so far. This PR begins to address it by adding a set of
standard error responses we can attach to all our API calls.

At a future point, we can add specific error types where necessary
(although we should not hurry to do that; it is best done very late.)

## Test Plan

Checked that Stainless SDK generation succeeds.

docs/openapi_generator/generate.py

@@ -55,6 +55,7 @@ def main(output_dir: str):
                 a set of endpoints and their corresponding interfaces that are tailored to
                 best leverage Llama Models.""",
             ),
+            include_standard_error_responses=True,
         ),
     )