scheduler: log errors returned by task.Manager.LaunchTask().
scheduler: log errors returned by task.Manager.LaunchTask().

This helps debug task managers such as gitiles trigger.

R=9f116ddb1b24f6fc1916a676eb17161b6c07dfc1@chromium.org

Bug: chromium:748234
Change-Id: Ic29345df6188410378608cba00a221abce8dd9a2
Reviewed-on: https://chromium-review.googlesource.com/722060
Reviewed-by: Vadim Shtayura <9f116ddb1b24f6fc1916a676eb17161b6c07dfc1@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