Warn/error in code path for Garmin protocol device that report bogus times

[GPSBabelDeveloper]

Note to self. I did most of the (painful) analysis in a recent bug report, but we really shouldn't be staring at hex dumps of protocol blabber to find that Garmin's firmware is making up times.

Moving that code path to gb::DateTimes would help, but jeeps has time_t's everywhere and is notoriously difficult/expensive to rework. We should at least hock up an error in the case we know we're going to blow out a time_t