The rationale for not being able to unmount a cache defaulting to a task failure rather than task exception is described in bug 1527799 comment 5. In this case, it is indeed the fault of the worker, and should be a task exception rather than failure, but this issue is being solved in bug 1526311.
Bug 1528422 Comment 2 Edit History
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The rationale for not being able to unmount a cache defaulting to a task failure rather than task exception is described in bug 1527799 comment 5. However, for the issue raised in this bug, the inability to persist the cache is indeed the fault of the worker, not the task, and therefore should be a task exception rather than failure. But this particular failure state is a consequence of bug 1526311.
The rationale for not being able to unmount a cache defaulting to a task failure rather than task exception is described in bug 1527799 comment 5. However, for the issue raised in this bug, the inability to persist the cache is indeed the fault of the worker, not the task, and therefore should be a task exception rather than failure. However, this goes away when bug 1526311 is fixed.