Recreate d.sentry if d.add_unit called after d.setup

Fixes https://bugs.launchpad.net/amulet/+bug/1296780

Signed-off-by: Tim Van Steenburgh <tvansteenburgh@gmail.com>
Use mock environment in new test.

Signed-off-by: Tim Van Steenburgh <tvansteenburgh@gmail.com>
Fail fast if unit goes to error state

Make Talisman.wait_for_status() raise immediately if a unit goes
to error state.

Signed-off-by: Tim Van Steenburgh <tvansteenburgh@gmail.com>
Merge pull request #29 from tvansteenburgh/lp1296780

Recreate d.sentry if d.add_unit called after d.setup