run_remote_tests.sh: make a failure message more meaningful

New message is more suggestive towards the correct approach to
build_packages at least once before one cros_workon starts and tries
running tests.

BUG=none
TEST=none

Change-Id: I5df8e50b25d3838025223c76d3633053a3bdafd9
Reviewed-on: https://gerrit.chromium.org/gerrit/22572
Tested-by: Zdenek Behan <zbehan@chromium.org>
Reviewed-by: Chris Sosa <sosa@chromium.org>
Commit-Ready: Zdenek Behan <zbehan@chromium.org>
1 file changed