Suppress a compiler warning under OpenVMS; time_t is unsigned on (at least)

Suppress a compiler warning under OpenVMS; time_t is unsigned on (at least)
the more recent versions of that platform, so we use the value (time_t)(-1)
as the error value.  This is the type used in the OpenVMS documentation:

http://www.openvms.compaq.com/commercial/c/5763p048.htm#inde

This closes SF tracker bug #404240.

Also clean up an exception message when detecting overflow of time_t values
beyond 4 bytes.