PSC-STM-A4: Add failure handling for invalid timepoint

[tiredpixel]

Since the records in the stream do not last forever, it is possible that a timepoint may be used from storage which is invalid, due to age. In this case, the stream client should catch the error and should act the same way as on first start where timepoint is null.

If the timepoint being used is invalid, an error should be logged somewhere (eg Rollbar), as it is a sign some data may have been missed.

Estimate: 4 hours

[tiredpixel]

I'm not sure we're actually using Rollbar in Register 2. This needs to be checked, and an approach agreed upon for this and any other failures.

[tiredpixel]

The approach I've chosen is to log, reraise, and crash the program if the timepoint is invalid. This allows the failure to be apparent to monitoring, as well as to leave program restarts to whichever process is overseeing it.

I haven't considered Rollbar or an alternative, here, since that relates to logging and exceptions more generally. This might be addressed specifically during deployment in #246 and #247 .