cli: Add CLI#session, DRYer exception guarding.
cli: Add CLI#session, DRYer exception guarding.

Commands can now access the session ensured by CLI#ensure_session!
via CLI#session, eliminating the need to manually instantiate
the KBSecret::Session instance. If ensure_session! is not called,
CLI#session will be nil (what else?).

Similarly, segments of code that are likely to throw an exception
can now be guarded via CLI#guard, which takes a parameterless
block and returns its value (while absorbing errors and converting
them into an early program exit).

woodruffw/kbsecret,kbsecret/kbsecret,kbsecret/kbsecret