[pct] Allow compilers to implement a .backtrace method to produce their own format of backtrace to print in the event of an uncaught exception. If not, we just fall back to Parrot's own handling.
[pct] Allow compilers to implement a .backtrace method to produce their own format of backtrace to print in the event of an uncaught exception. If not, we just fall back to Parrot's own handling.

git-svn-id: 6e74a02f85675cec270f5d931b0f6998666294a3@47113 d31e2699-5ff4-0310-a27c-f18f2fbe73fe

ashgti/parrot,ashgti/parrot,ashgti/parrot,ashgti/parrot,ashgti/parrot,ashgti/parrot,ashgti/parrot