Forgive the case when a whiteout file trying to cover nonexistent path

[yiranwang52]

This error happens sometimes with python package's .dist-info or .pth files. For example, installing aiohttp package in a conda environment, then when applying cache of layer, this error message happens often:

[error] 2021-03-09 17:36:12.468002: failed to execute build plan: execute stage: build stage 0: build node: apply cache: untar reader: add hdr from tar to layer: update memfs with file /home/ray/anaconda3/lib/python3.7/site-packages/.wh.aiohttp-3.7.4.dist-info: whiteout nonexistent path /home/ray/anaconda3/lib/python3.7/site-packages/aiohttp-3.7.4.dist-info

Those files were probably created or removed asynchronously.
It doesn't hurt to ignore this case; Change to a warning message instead.

[sema]

Discussed this change with @yiranwang52

We both agree that this is likely related to async operations being done on these files, which Makisu does not currently handle well.

We don't fully understand how this may then manifest itself exactly like this when re-applying the cache layers. There may exist a race in how the cache layers are calculated when we have async operations.

Approving this to unblock the builds failing due to this, as the error in itself is not dangerous.