Allow an algorithm to report messages to TPL

[alastair]

Related to #17

ControlAction has an error field which allows us to set a message when we set the actionStatus to failed. This is useful for debugging and reporting the cause of an error.
It would be nice for an algorithm to be able to report this error message back to the TPL, but I'm not sure what the best way to pass this data could be.
One option is to use stderr in the algorithm, if the return code of the algorithm is non-0 and stderr is non-empty then we can add that to the error field of the ControlAction.