systemd provider should expose systemd command output

[pclalv]

Use Case

When debugging an issue with a failing systemd command (start, restart, and stop), it helps to know the systemd command's output.

Describe the Solution You Would Like

When the systemd provider's methods start, restart, and stop raise a Puppet::Error, I want that error to include the systemd command's output.

Describe Alternatives You've Considered

If the user wants this output, they have little recourse but to inspect the gem's source code and modify it in-place.

Additional Context

#prepare_error_message already accepts an exception, but it does nothing with it. This exception includes the systemd command's output. Implementing this improvement is a matter of figuring out where to insert exception.message in the error_result string.