vpython: more logging when incomplete virtualenv dir can't be removed.
vpython: more logging when incomplete virtualenv dir can't be removed.

R=iannucci, smut

Bug: 869227
Change-Id: I2095c781c7f2ead94bd309079803ab44ec63273d
Reviewed-on: https://chromium-review.googlesource.com/c/infra/luci/luci-go/+/1876641
Commit-Queue: Andrii Shyshkalov <a30c74fa30536fe7ea81ed6dec202e35e149e1fd@google.com>
Commit-Queue: smut <95deb7aa15d4c615299e47e7688551456568b1fb@google.com>
Auto-Submit: Andrii Shyshkalov <a30c74fa30536fe7ea81ed6dec202e35e149e1fd@google.com>
Reviewed-by: smut <95deb7aa15d4c615299e47e7688551456568b1fb@google.com>
Reviewed-by: Robbie Iannucci <40f3d43a28ebae3cb819288542e1c84d73d962d5@chromium.org>

luci/luci-go,luci/luci-go,luci/luci-go,luci/luci-go,luci/luci-go,luci/luci-go