Add missing error check.
Add missing error check.

Change-Id: I6fcf6d5d50333ff0c205fd87cdb9430f1bc02c43

KevinPike/client_golang,stapelberg/client_golang,jimmidyson/prometheus-client_golang,m3db/prometheus_client_golang,mattkanwisher/client_golang,asbjornenge/client_golang,fabxc/client_golang,prometheus/client_golang,telendt/client_golang