Alternative cancellation messages with "internal" cancellation

All CANCELLED statuses now have the same error message regardless of the source of the cancellation. There are many places in the code where we propagate "CANCELLED" statuses around the place internally, e.g. within scans, for reasons unrelated to user-initiated cancellation. Those should not leak

IMPALA-7418 was much harder to debug than necessary because it wasn't clear where the CANCELLED status came from.