crash: do not call list-proxies if it does not exist.

Additionally, this adds logging for list-proxies call failures.

BUG=chromium:396033
TEST=unittests, test_that logging_UserCrash

CQ-DEPEND=CL:275076

Change-Id: I0515e75476bdc7a0ace8ebc8318e82c337a87374
Reviewed-on: https://chromium-review.googlesource.com/275077


Reviewed-by: Mike Frysinger <vapier@chromium.org>
Commit-Queue: Daniel Colish <colish@chromium.org>
Tested-by: Daniel Colish <colish@chromium.org>