This is a flag that functions in the execution environment can return to
report on the status of the operation. This way they can report an error
without forcing the entire invocation to shut down.
Marked the old versions of PrepareFor* that do not use tokens as
deprecated and moved all of the code to use the new versions that
require a token. This makes the scope of the execution object more
explicit so that it will be kept while in use and can potentially be
reclaimed afterward.
