Fix LocalStore.GetChunk dropping read errors

[folbricht]

LocalStore.GetChunk() only checked the error from os.ReadFile with os.IsNotExist(). Any other read failure — permission problems, I/O errors, truncated reads — was silently ignored and the nil or partial buffer handed to NewChunkFromStorage():

• With verification enabled, the caller saw a misleading ChunkInvalid instead of the real error (and a RepairableCache would then treat it as missing and re-fetch, hiding e.g. a permission problem indefinitely).
• With SkipVerify set (--skip-verify, or the pull command), the store returned a chunk holding partial data with no error at all.

Return the underlying error instead. The new test uses a directory in place of the chunk file to trigger a non-IsNotExist read error on all platforms; it fails against the previous code (returned a bogus chunk with nil error under SkipVerify).