all: use errgo.

This almost entirely an automated change, made by running
the errfix comment. (code.google.com/p/rog-go/exp/cmd/errfix)
all: fix misplaced comments
Merge pull request #47 from rogpeppe/018-use-errgo

all: use errgo.

This almost entirely an automated change, made by running
the errfix comment. (code.google.com/p/rog-go/exp/cmd/errfix)