Vine: Specific error cases for vine_fetch_file

[dthain]

Proposed Changes

Modify vine_fetch_file to set errno according to various cases:

• ENOENT - file has not yet been created, no context expected.
• EAGAIN - file has been created, but no replica currently available, try again
• EIO - some other unexpected failure mode, data no longer available

Fixes #3750

Merge Checklist

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