Unpowered buildings can't cancel production #96

See bwapi/bwapi#918

JBWAPI, like the BWAPI client before it, rejects commands to unpowered units: https://github.com/JavaBWAPI/JBWAPI/blob/develop/src/main/java/bwapi/Unit.java#L3189

Fixing it on the JBWAPI side isn't enough -- the BWAPI server will reject the command too -- but is a good start. Like I wrote on the other issue, I do not think isPowered needs to be checked at all, or at most should be conditioned on whether the command is a Cancel command.