Check, if a service is registered

[buschtoens]

Currently {{service "unknown-service"}} throws an assertion during render.

Users may want to check, whether a service exists. For this we could either add a second helper, e.g. {{service-exists "unknown-service"}} that returns true / false, or we could add an optional named parameter {{service "unknown-service" throwIfUnknown=false}} and return undefined, if the service is unknown.

I'm not yet convinced that this is a good idea and I did not have the need for it yet. Maybe this kind of logic should be kept in JS.

Looking for opinions / suggestions!