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.

llama_stack/apis/datatypes.py

@@ -5,6 +5,9 @@
 # the root directory of this source tree.
 
 from enum import Enum
+from typing import Optional
+
+from pydantic import BaseModel
 
 from llama_stack.schema_utils import json_schema_type
 
@@ -33,3 +36,20 @@ class Api(Enum):
 
     # built-in API
     inspect = "inspect"
+
+
+@json_schema_type
+class Error(BaseModel):
+    """
+    Error response from the API. Roughly follows RFC 7807.
+
+    :param status: HTTP status code
+    :param title: Error title, a short summary of the error which is invariant for an error type
+    :param detail: Error detail, a longer human-readable description of the error
+    :param instance: (Optional) A URL which can be used to retrieve more information about the specific occurrence of the error
+    """
+
+    status: int
+    title: str
+    detail: str
+    instance: Optional[str] = None