stembuild: replace custom messangers

- extract constants
- capture error from Write() and panic if present
- message used fantasy (2035) version instead of actual (2019)
- extract common `write` func
- extract messenger from main()
- remove references to pivotal email
- packagers use Messenger not STDOUT/STDERR

Use base messenger methods and spy on output rather than fake messengers
with specific output funcs that are mocked.

Author: aramprice