Offloaded bootstrap code to the main codebase.

Specifically:

* Offloaded print_exception_message() and
  print_exception_exit_status() to new component "zsh/exception".
* Replaced say() and cry() with simple hard-coded print() statements.

Additional offloadings to follow, time and the Green Goddess affording.
Documented additional subtle issues in "main", to be similarly fixed...