#42 Exception trace-back is in English

[Mark Lentczner]

At present, the Exception class accepts a single text string for each
layer of the stack trace back. The VirtualMachine adds a string of
the form:

computeThing sent to /home/mark/stuff/foo(/library/extras/gizmo)
found in /library/extras/computable

This lists (in order from left to right):
1- the messsage sent
2- the receiver (self)
3- the prototype of the receiver
4- the object where the message was found (this)

Alas, this line is formatted in English. Either a) the format should
be all symbols, like msg >> self(proto)/this or b) Exception should
take a structure for each trace back level, so that this can be
formatted later on.