gobs: error cleanup part 1.

gobs: error cleanup part 1.
Remove err from the encoderState and decoderState types, so we're
not always copying to and from various copies of the error, and then
use panic/recover to eliminate lots of error checking.

another pass might take a crack at the same thing for the compilation phase.

R=rsc
CC=golang-dev
https://golang.org/cl/2660042