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/pyopenapi/options.py

@@ -35,6 +35,7 @@ class Options:
     :param error_wrapper: True if errors are encapsulated in an error object wrapper.
     :param property_description_fun: Custom transformation function to apply to class property documentation strings.
     :param captions: User-defined captions for sections such as "Operations" or "Types", and (if applicable) groups of extra types.
+    :param include_standard_error_responses: Whether to include standard error responses (400, 429, 500, 503) in all operations.
     """
 
     server: Server
@@ -52,6 +53,7 @@ class Options:
     error_wrapper: bool = False
     property_description_fun: Optional[Callable[[type, str, str], str]] = None
     captions: Optional[Dict[str, str]] = None
+    include_standard_error_responses: bool = True
 
     default_captions: ClassVar[Dict[str, str]] = {
         "Operations": "Operations",