Update error reporting

[chengsun]

Right now error reporting is done by calling ui.Error to print a red message. However, this message can easily get lost amongst the noise of any other builders running simultaneously. This is why other builders also do a state.Put("error", err) before returning multistep.ActionHalt -- this way Packer can repeat the error at the end, when all the builders have terminated.

(I avoided doing this at first because it would introduce a huge amount of additional code noise, so if there's a clean way to do this then that would be good.)