fix: make early check if users filesystem have a mountpoint at /<user>/files

[Antreesy]

• Resolves: follow-up to fix: don't skip scanner users filesystem if they have a mountpoint at /<user>/files/ #49887

Summary

Scanner checks both /<user> /<user>/files directories:

1. If it exists on cache and on disk
2. If it is writable
3. If by chance path is overwritten with ext mountpoint

1 can still fail, if overwritten path of 3 is cached, and default dir doesn't exist, so 3 should be check first

UPD: another issue appeared while testing, added $relativePath to NotFoundException

Checklist

• Code is properly formatted
• Sign-off message is added to all commits
• Tests (unit, integration, api and/or acceptance) are included
• Screenshots before/after for front-end changes
• Documentation (manuals or wiki) has been updated or is not required
• Backports requested where applicable (ex: critical bugfixes)

[Antreesy]

Questions:

• was there a reason to not check it first in initial PR?
• will the periodic file scans (triggered by cron) work with that consideration?