added linenumber method to ExceptionPacket, started adding backtrace
merge; change in comments only
provided access to excpetion packet data via Grace methods. Changed direction of backtrace so that most recent frames come first.  Removed die and replaced calls with gracedie (the two functions being identical)
changed limits on backtrace to use calldepth, and added explicit int to float conversion
Merge branch 'exceptions'

Conflicts:
	gracelib.c
	mgcollections.grace
Conflicts fixed by hand by Andrew