Clarify missing venv dir error message

BUG=None
TEST=None

Change-Id: I780489c245115dfd96f44710a744bfb8235ee25f
Reviewed-on: https://chromium-review.googlesource.com/621595
Commit-Ready: Allen Li <ayatane@chromium.org>
Tested-by: Allen Li <ayatane@chromium.org>
Reviewed-by: Shuhei Takahashi <nya@chromium.org>

bin/python_venv

diff --git a/bin/python_venv b/bin/python_venv
index 198ebf5..6c50d53 100755
--- a/bin/python_venv
+++ b/bin/python_venv
@@ -57,7 +57,7 @@
 readonly extra_imports_dir=$(realpath ../venv)
 if [[ ! -d "${extra_imports_dir}" ]]; then
     cat <<EOF >&2
-ERROR: venv directory is not found at ${bin_dir}/..
+ERROR: ${bin_dir}/../venv does not exist
 See infra_virtualenv/README.md for details.
 EOF
     exit 1