Reject out-of-range output when converting to TS_MW

TS_MW is a 14-character string "YYYYMMDDHHIISS", and thus cannot
represent timestamps earlier than 00000101000000 or later than
99991231235959.

MWTimestamp should throw an exception if asked to represent out-of-range
times in this format, rather than returning invalid values that are
likely to be truncated by the database.

Bug: T51580
Change-Id: I744e446356f3ed9193dfaaaec5dc81c611dab4a3