when an exception is catched, just exit since we already reported the error
check for endTime() and startTime() validity when returning for duration()

if invalid, returns 0 seconds as duration()
better error report when incomplete links are found
check valid properties when description() is called for all entities
call duration() instead, better error check
allow PajeSimulator to ignore incomplete links (not recommended)

note:
- this will be used by pj_dump to dump even if incomplete links
are in the trace file
new option to pj_dump to ignore incomplete links ( -z )