exec-script actions to have a description

[tbatchelli]

In order to provide the most useful information in explain-plan and explain-session, it would be good that all actions had a meaningful description.

exec-checked-script already allows for this.

This description should be also part of the :results in the session (it isn't now, neither for exec-checked-script)

[hugoduncan]

Not sure what the best way of doing this is without breaking existing exec-script usage.