[server] Add Server.Fatal(...).
[server] Add Server.Fatal(...).

It is roughly like log.Fatal(...) or panic(...), but logs to our logger and
understand our annotated errors. Useful when reporting non-recoverable errors
during the server startup.

R=a30c74fa30536fe7ea81ed6dec202e35e149e1fd@chromium.org
BUG=959427

Change-Id: Iaf265206e6c938cf77e5a1fe9abdc90de7e91208
Reviewed-on: https://chromium-review.googlesource.com/c/infra/luci/luci-go/+/1666518
Reviewed-by: Andrii Shyshkalov <a30c74fa30536fe7ea81ed6dec202e35e149e1fd@chromium.org>
Commit-Queue: Vadim Shtayura <9f116ddb1b24f6fc1916a676eb17161b6c07dfc1@chromium.org>

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