[task_scheduler] Use errors with context where possible
[task_scheduler] Use errors with context where possible

I had a hard time tracking down an error today and I think the extra
context would have made it easier.

Change-Id: I4120d8e20a35c42ff4d30df9877c0e34477c3328
Reviewed-on: https://skia-review.googlesource.com/c/buildbot/+/537979
Commit-Queue: Kevin Lubick <7cdab2cfab351f23814786ba39716e90eed69047@google.com>
Reviewed-by: Eric Boren <0e499112533c8544f0505ea0d08394fb5ad7d8fa@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