Improve the error message when using opt photons as primary

[paolafer]

Optical photons' trajectories are not stored in the simulation when using DefaultTrackingAction, to avoid creating huge output files. If one wants to save them, OpticalTrackingAction should be used instead, only for small productions and debugging.
On the other hand, DefaultEventAction tries to access the trajectory container, to look for the hits associated with every particle and make a cut on the total deposited energy.
As a consequence, the combination of DefaultTrackingAction and DefaultEventAction produces a segmentation fault in simulations of optical photons as primary particles, since no other particles are present in the simulation.
For the same reason, using DefaultEventAction but not DefaultTrackingAction in standard simulations also causes a segmentation fault.
This PR tries to improve the error message to cover these two cases, which are the most common causes of that particular segmentation fault.

[Aretno]

The error message is now much clearer and points directly to the issue. Nice!