fix: Skip letter heads if no permission (backport #26689)

[mergify]

#26563 This cases boot to fail for users if they don't have letterhead access.

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This is an automatic backport of pull request #26689 done by Mergify.

[mergify]

Cherry-pick of 63a13d0 has failed:

On branch mergify/bp/version-15/pr-26689
Your branch is up to date with 'origin/version-15'.

You are currently cherry-picking commit 63a13d0aff.
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Unmerged paths:
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	both modified:   frappe/boot.py

no changes added to commit (use "git add" and/or "git commit -a")

To fix up this pull request, you can check it out locally. See documentation: https://docs.github.com/en/pull-requests/collaborating-with-pull-requests/reviewing-changes-in-pull-requests/checking-out-pull-requests-locally