[vpython] more context in errors
[vpython] more context in errors

Add more context to error message in order to help debug obscure failure
case on CrOS.

TBR=iannucci@chromium.org
BUG=chromium:808434
TEST=None

Change-Id: I59974086c6aeb469aee9aa1dda85be35412ba570
Reviewed-on: https://chromium-review.googlesource.com/910197
Commit-Queue: Daniel Jacques <8400006a9338a0893dca0101ad49838955bd6b2c@chromium.org>
Reviewed-by: Daniel Jacques <8400006a9338a0893dca0101ad49838955bd6b2c@chromium.org>

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