feat: add OasstErrorResponse to protocols

Using a shared protocol to serialize the error in the backend
allows clients to use that same protocol to deserialize it.

Changes to this protocol will be caught in tests.

backend/main.py

@@ -26,8 +26,13 @@ app = fastapi.FastAPI(title=settings.PROJECT_NAME, openapi_url=f"{settings.API_V
 @app.exception_handler(OasstError)
 async def oasst_exception_handler(request: fastapi.Request, ex: OasstError):
     logger.error(f"{request.method} {request.url} failed: {repr(ex)}")
+
     return fastapi.responses.JSONResponse(
-        status_code=int(ex.http_status_code), content={"message": ex.message, "error_code": ex.error_code}
+        status_code=int(ex.http_status_code),
+        content=protocol_schema.OasstErrorResponse(
+            message=ex.message,
+            error_code=OasstErrorCode(ex.error_code),
+        ).dict(),
     )