scripts: Make cros_generate_stacks_bvt user-friendlier.
scripts: Make cros_generate_stacks_bvt user-friendlier.

This adds support for using debug tarballs located on the
local machine instead of only ones on remote servers.  (If
the first argument is a local file, we use it; otherwise, we
try to download it.)

It also makes us download the (tiny) minidumps before the
(huge) tarball so we can bail out quickly if the minidump
URL is wrong.

BUG=chromium-os:19565
TEST=manual: ran it successfully with both local and remote tarballs and checked that it fails early when given an invalid minidump path

Change-Id: Ie427b50e4b7e37c6c81a1137eb34a28e25f32c8e
Reviewed-on: http://gerrit.chromium.org/gerrit/8910
Tested-by: Daniel Erat <69d56254d3d393520d06e9838c23c147a5fee7d9@chromium.org>
Reviewed-by: Steven Bennetts <d2bc269fbc7f1691ea11e20b3ea986cb219ed714@chromium.org>
Reviewed-by: Thieu Le <8cae94ed37a002427b56783feab9ab75ab7aa0b8@chromium.org>
Commit-Ready: Daniel Erat <69d56254d3d393520d06e9838c23c147a5fee7d9@chromium.org>

zhang0137/scripts,mjg59/scripts,vmware/coreos-scripts,trnubo/scripts,mjg59/scripts,glevand/coreos--scripts,bcwaldon/coreos-scripts,BugRoger/scripts,sigma/coreos-scripts,andrejro/coreos--scripts,mischief/scripts,coreos/scripts,sigma/coreos-scripts,endocode/scripts,nicescale/scripts,fivethreeo/scripts,gtank/scripts,bcwaldon/coreos-scripts,andrejro/coreos--scripts,crawford/scripts,crawford/scripts,marineam/coreos-scripts,fivethreeo/scripts,mischief/scripts,nicescale/scripts,andrejro/coreos--scripts,trnubo/scripts,bcwaldon/coreos-scripts,BugRoger/scripts,coreos/scripts,mischief/scripts,smilart/scripts,gtank/scripts,crawford/scripts,gtank/scripts,marineam/coreos-scripts,smilart/scripts,vmware/coreos-scripts,zhang0137/scripts,glevand/coreos--scripts,zhang0137/scripts,smilart/scripts,vmware/coreos-scripts,nicescale/scripts,mjg59/scripts,endocode/scripts,sigma/coreos-scripts,trnubo/scripts,fivethreeo/scripts,BugRoger/scripts,endocode/scripts,marineam/coreos-scripts