add exception handler and respond with template to throttling

[ax-sc]

Fine-grained rate limits have been introduced with 52b71e2 to better reflect valid usage patterns during public events. Throttling should usually not be a problem in daily usage patterns but (not limited to but especially during public events) it would still be useful to provide more context to users by introducing a custom error handler with a template response containing a hint about the throttling instead of a generic server error.