Missing error handling in the snapshot creation endpoint

[BGehrels]

The snapshot creation endpoint treats the filter parameter (added in #51) as an opaque string and hands it over to the host system. Sadly, it does not provide a mechanism to report invalid filter parameters and transform them into a reasonable status code/error message. So, at the moment, errors thrown by a SnapshotEventGenerator will be propagated as HTTP Status 500 to the caller, sometimes with stack trace information.