powerman: return textual error on on/off fail

[chu11]

In the early days of powerman, a power control (sort of) could only catastrophically fail (e.g. network down) or just work.

But since IPMI support, "semi normal" errors could occur, such as authentication (i.e. username/password not setup correctly), network timeouts, or even "busy" errors (i.e. service processor busy) can be half expected to happen once in awhile.

powerman cannot relay such textual errors back to the user and often just says "command completed successfully".

Not sure if can be handled during #74, probably a bigger todo item.

[garlick]

I think this can be combined with #79 so I'll close this one and propose a concrete plan over there.