[milo] fix double bug in handling not found Gerit CLs.
[milo] fix double bug in handling not found Gerit CLs.

If CL is not found, Milo previously attempted to cache such result.
During prior refactoring, a bug was introduced which caused panic.
Additionally, previously invisible CL may become visible, so
caching invisibility is incorrect.

R=2190fe9a39b4920ec7bd59c848ec5cccd163056f@chromium.org

Bug: 846378
Change-Id: If79665e309c0a086877ec1cb0d04c55ff2478aa7
Reviewed-on: https://chromium-review.googlesource.com/1073998
Reviewed-by: Ryan Tseng <2190fe9a39b4920ec7bd59c848ec5cccd163056f@chromium.org>
Commit-Queue: Andrii Shyshkalov <a30c74fa30536fe7ea81ed6dec202e35e149e1fd@chromium.org>

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