Add some validation to OpticalStimulationStatus

[slagernate]

After some discussion with @chrisl-science, decided it would be a smoother experience for users if there were a message to inform them if their LED-on duration (duration_us in the optical stim frame) is valid (i.e. compatible with frame rate and # of rows (for passive display)).

Currently we have:

message OpticalStimulationStatus {
  // How many frames have we written?
  uint64 frames_written = 1;
}

Could add a bool (e.g. last_frame_valid) or could update/add ``valid_frames_written` or something similar.