Beginning of unregister behavior.
Watch last error and send terminated message when plugin exits.
Test optional exit filter/output behavior
Prevent leaking packs with optional exit
No more special casing of SandboxFilters stopping.

Handle errors from SandboxFilters better as well.
Update tests for new sandbox filter error behavior
Update canExit config to use plugin default.

Fix some comments.
Use custom TerminatedError type for sandbox_filter
Update docs with can_exit behavior.
Remove unncessary pConfig args, and use foRunner.pConfig

Skip unnecessary plugin wrapper check in foRunner.Starter()
Merge pull request #1015 from ecnahc515/optional_exit

Filters/Outputs can optionally exit without shutting down heka