cmd/go: Check error from SWIG link step.
cmd/go: Check error from SWIG link step.

LGTM=iant
R=iant
CC=golang-codereviews
https://codereview.appspot.com/85070043

Committer: Ian Lance Taylor <87e9c6d529889242b7e184afb632328636553ab4@golang.org>

apparentlymart/golang,apparentlymart/golang,apparentlymart/golang,apparentlymart/golang,apparentlymart/golang,apparentlymart/golang,apparentlymart/golang