cmd/coordinator: check error getting a token source
cmd/coordinator: check error getting a token source

This bit me a few weeks ago when I had some local unsubmitted
oauth2/google changes which failed, but the coordinator kept trying
to run with a nil token source.

Change-Id: I351e626e5c28fb8982786fbb1ba7a8a09d9bb86d
Reviewed-on: https://go-review.googlesource.com/18800
Reviewed-by: Andrew Gerrand <395a7d33bec8475c9b83b7d440f141bcbd994aa5@golang.org>

golang/build,golang/build,golang/build,proppy/build,proppy/build,proppy/build,proppy/build,golang/build