Return stderr and stdout if any stderr results from command execution

[bxeno]

If any stderr results from a command execution, only stderr is returned as the command output to the caldera api server.

This can affect troubleshooting of the command execution as useful information may be present in stdout.

Both stderr and stdout are returned if the command execution times out.

I propose that the same occurs if any stderr results from command execution.

I have raised a PR #419 as a proposed solution.