holochain -c <config> doesn't exit with a non-zero exit code on parsing failure

[pjkundert]

If there is an error in the configuration file, holochain should exit with a non-zero exit code, so that this failure can be detected by scripts:

$ holochain -c conductor-agent-0.toml
Using config path: conductor-agent-0.toml
Error while trying to boot from config: IoError("Could not serialize toml: invalid number at line 60")
$ echo $?
0