Report a special error code when assertion has a bad timestamp.

This implements a special error status for invalid-timestamp, as requested in #17.  It also lays the groundwork for reporting invalid-generation errors as documented in https://bugzilla.mozilla.org/show_bug.cgi?id=958462

I had to drop the use of cornice_error helper in order to customize the 'status' field; we should look at syncing this into upstream cornice at some point, per discussion in https://bugzilla.mozilla.org/show_bug.cgi?id=792674.  But getting this live needs to take priority.

@ametaireau r?