powerman: support new --diag option

[chu11]

Built upon #161

Now that the new setresult statement is available in #161, it's trivial to report back why individual power operations failed.

e.g.

> pm --diag -1 t[0-15]
t[7,15]: error
Command completed with errors

Since I don't want default output to change too much I have this dispalyed via a new --diag option to powerman. Can debate option name of course.

Will be super helpful as we work towards completing #128

[chu11]

oh crap, i pushed to chaos/powerman, not chu11/powerman ... i'll re-open with a new PR

[mergify[bot]]

⚠️ The sha of the head commit of this PR conflicts with #163. Mergify cannot evaluate rules on this PR. ⚠️