localstorage: Demote reservation stat error to debug

[magik6k]

This can be a bit spammy, and can happen normally, for example when finalizing the sector:

• We'll reserve space for the sealed sector and cache,
• We'll start fetching sealed data
• Cache will not start to be fetched until we fetch the sealed sector file
• Since that takes a while, we'll spam this error when periodically checking storage health / getting disk usage when scheduling tasks

[arajasek]

Needs tests.