upload_command_stats: silently move on if host cannot be determined.
upload_command_stats: silently move on if host cannot be determined.

If the cros_build_lib.GetHostName function fails to determine the
name of the current host then the upload_command_stats script will
now silently skip the upload of stats, rather than crashing.

BUG=chromium-os:32171
TEST=`build_packages --board=x86-generic` still uploads for me
TEST=Thought traced stack in bug report, high degree of confidence
that this fixes the problem.
TEST=Locally edited cros_build_lib.GetProjectUserEmail to return None, then
verified that new warning gets printed and no stats uploaded.

Change-Id: I31154fe8ac89def0f3899a8be62e3c454f9f7eae
Reviewed-on: https://gerrit.chromium.org/gerrit/26160
Commit-Ready: Matt Tennant <46e612415b446ccde19638e4035892001c237942@chromium.org>
Reviewed-by: Matt Tennant <46e612415b446ccde19638e4035892001c237942@chromium.org>
Tested-by: Matt Tennant <46e612415b446ccde19638e4035892001c237942@chromium.org>

coreos/chromite,zhang0137/chromite,coreos/chromite,chadversary/chromiumos.chromite,zhang0137/chromite,zhang0137/chromite,coreos/chromite,bpsinc-native/src_third_party_chromite,bpsinc-native/src_third_party_chromite,bpsinc-native/src_third_party_chromite,chadversary/chromiumos.chromite