Fix error response

The `error` field is not valid for an API Gateway response. As noted in the AWS docs below, when an error response contains invalid fields, it will always return a `502` error code with a body of `{"message": "Internal server error"}`. This PR updates the error response fields to contain valid fields and a more sensible response message.

- https://docs.aws.amazon.com/lambda/latest/dg/services-apigateway.html#services-apigateway-errors

lambda-function.js

@@ -28,7 +28,7 @@ async function lambdaFunction(probot, event, context) {
   } catch (error) {
     return {
       statusCode: error.status || 500,
-      error: "ooops",
+      body: "Error processing GitHub Event",
     };
   }
 }