All consul commands use proper exit codes

[ethanrowe]

All existing commands should have meaningful exit codes:

• 0 for normal exit
• non-zero for non-standard exit

The heartbeat should probably exit with 255 if the agent isn't running (meaning it can't get information with the consul info call), and 0 otherwise.

The runner should probably exit with consul's exit code if consul exits with non-zero status. And otherwise exit with 0.