fix(fleet): exit if systemd reports failure

If you run `deisctl start platform` and one of the units decide to
fail halfway through deployment, deisctl will continue to run until
the unit shows "running", which means it'll effectively run forever.
This lets `start` bail out if the unit fails to start.

example:

```
><> deisctl start platform
● ▴ ■
■ ● ▴ Starting Deis...
▴ ■ ●

Launching data containers...
deis-database-data.service: exited
deis-registry-data.service: exited
deis-logger-data.service: exited
deis-builder-data.service: exited
Data containers launched.
Launching service containers...
deis-logger@1.service: failed
Error: systemd reported unit in failed state
```