Pass failed build error data back in heartebeat

[dowdt]

This way the user can be notified what error occurred and (if they're savvy with nix) they might be able to fix it.

The simple version of this would just be displaying the failed Nix build error log.

To implement:

• Add "last build error" field to heartbeat (Encoded as base64 or compressed pls).
• Add "is failed build" field to the heartbeat (Should be possible since failed builds shouldn't restart the admin service).

Undecided if this should be reflected in the dpl status at all.
Undecided if we should only post once and send this as a notification instead. (Dpl notifications are currently not implemented).