Throw exception in FlatbufferReader if key not found.

Created by: mchorazak

FlatbufferReader::ptr &find(std::string const &FlatbufferID) should throw an exception if key not found

https://github.com/ess-dmsc/kafka-to-nexus/blob/9cf7b41ebeacec0b2ac1dbb55aecce9b4c307797/src/FlatbufferReader.h#L38-L39