chore: don't log non-error message at error level

When walking the tree for creating the search index, ignore "not found"
errors. Search indexing happens asynchronously so the underlying space,
file or folder might just not exist anymore when the indexing starts.
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Internal Server Error - Gitea: Git with a cup of tea
500 Internal Server Error

Gitea Version: 1.28.0+dev-477-g8b6ad49a5f