Specify behavior of default exception handler

[cowan]

If an exception is caught and leaves the current dynamic extent, obviously the after thunk must be run, but an uncaught exception has no semantics and is basically reverting to "is an error" semantics, i.e. nasal demon territory.

Possibly we should tighten this up in the standard, i.e. specify that there is a default exception handler which enters a continuation outside the extent of the whole program before exiting.

[cowan]

The WG voted to reject this proposal and leave the behavior unspecified.