buildlet: log on heartbeat failures
buildlet: log on heartbeat failures

should be rare now.

Change-Id: Icc4bfd13c8dfe8f2e189db819bc0d552f35fb3c9
Reviewed-on: https://go-review.googlesource.com/14731
Reviewed-by: Andrew Gerrand <395a7d33bec8475c9b83b7d440f141bcbd994aa5@golang.org>

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