[3.6] bpo-31178: Avoid concatenating bytes with str in subprocess error (GH-3066...

[3.6] bpo-31178: Avoid concatenating bytes with str in subprocess error (GH-3066) (#3388)

Avoid concatenating bytes with str in the typically rare subprocess error path (exec failed). Includes a mock based unittest to exercise the codepath.
(cherry picked from commit 3fc499bca18454b9f432b9b0106cab662bfeb549)