Exceptions: add exception-specific (command-sequence) fuzzer. (#12923)

* Exceptions: add exception-specific (command-sequence) fuzzer.

This fuzzer uses a description of a set of "scenarios", arbitr

Exceptions: add exception-specific (command-sequence) fuzzer. (#12923)

* Exceptions: add exception-specific (command-sequence) fuzzer.

This fuzzer uses a description of a set of "scenarios", arbitrarily
generated, to produce a specific kind of module that tests throw/catch
behavior. The module contains a chain of functions that invoke each
other; one will throw, and the rest may have catch clauses that do or
do not catch.

* Review feedback.

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