synthesizeTransientStats: add --timestampsFiles user option (and an added error check in MFDv5)

@karl-wette : A small generalization to the synthetic transient-Fstat code. The underlying architecture already handles gaps, so it was really just a question of adding the uvar.

Since I was copying code for this from makefakedata_v5, I've added to this MR a small second patch which makes that code also fail earlier if given an existing, but empty timestamps file, which so far fell through to some XLALFindSmallestValidSamplingRate() function before raising an error.