[Leasing Server] Handle case where leasing tasks are cancelled
[Leasing Server] Handle case where leasing tasks are cancelled

Bug: skia:
Change-Id: I9c47e417d2ab05acfcc8d00c3f95c194ba2da502
Reviewed-on: https://skia-review.googlesource.com/78260
Commit-Queue: Ravi Mistry <9fa2e7438b8cb730f96b74865492597170561628@google.com>
Reviewed-by: Kevin Lubick <7cdab2cfab351f23814786ba39716e90eed69047@google.com>

google/skia-buildbot,google/skia-buildbot,google/skia-buildbot,google/skia-buildbot,google/skia-buildbot,google/skia-buildbot,google/skia-buildbot,google/skia-buildbot