cbuildbot_stages.py: Make the pass/fail streak message more use friendly.
cbuildbot_stages.py: Make the pass/fail streak message more use friendly.

The pass/fail streak message wasn't very user friendly. It could print a
negative value which wouldn't really mean anything for anyone not familiar
with the implementation. This CL changes the message so it's a little less
cryptic.

BUG=chromium:345084
TEST=Ran chromite/buildbot/run_tests. Ran a remote trybot with both a passing
and failing build and verified that the message appeared correctly.

Change-Id: I921c47b92a5bffaa00d0156d1a25ef8ad9e00ec9
Reviewed-on: https://chromium-review.googlesource.com/197761
Reviewed-by: David James <f231dc574bbeb62604660cbc162d1b8642a0a3af@chromium.org>
Commit-Queue: Gabe Black <5c8aa1266c03186393d9e7b3b4e3b14239af8201@chromium.org>
Tested-by: Gabe Black <5c8aa1266c03186393d9e7b3b4e3b14239af8201@chromium.org>

chadversary/chromiumos.chromite,chadversary/chromiumos.chromite,bpsinc-native/src_third_party_chromite,bpsinc-native/src_third_party_chromite,bpsinc-native/src_third_party_chromite