fix(ssh.Execute): Handle ssh execution errors.
Merge pull request #241 from bcwaldon/drop-wantedby

Stop writing WantedBy into units
Merge pull request #234 from calavera/handle_ssh_execution_errors

fix(ssh.Execute): Handle ssh execution errors.
refactor(systemd): Store raw unit files
refactor(systemd): Drop fleet target
doc(unit): Add docstrings to SystemdUnitFile methods