More precisions required for runtime_error

[pomerlef]

There is two occurrences of this error in the code: throw std::runtime_error("Requested trajectory evaluator at an invalid time.");

• https://github.com/utiasASRL/steam/blob/develop/src/trajectory/SteamTrajInterface.cpp#L97
• https://github.com/utiasASRL/steam/blob/develop/src/trajectory/SteamTrajInterface.cpp#L119

It would be easier to debug if the faulty time would be reported and the limit that it breaks.