Add several tests to check consistency of stack traces.
The fix for [3610404] leads to a simplification in the implementation of
forward methods.
Make sure the errors raised by execution traces become errors raised by
the traced command, as documented.  Deletion of the traced command was
supressing that.
Make sure the errors raised by execution traces become errors raised by the
traced command, as documented. Deletion of the traced command was supressing
that.
merge trunk