[stable10] Convert StorageNotAvailable to Sabre one if can't get the child - #35707

Description

Backport of #35579 to stable10

Related Issue

https://github.com/owncloud/enterprise/issues/3369

Motivation and Context

return a 503 error code if the storage isn't available instead of a 500

How Has This Been Tested?

• test environment:
• test case 1:
• test case 2:
• ...

Screenshots (if appropriate):

Types of changes

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Database schema changes (next release will require increase of minor version instead of patch)
• Breaking change (fix or feature that would cause existing functionality to change)
• Technical debt
• Tests only (no source changes)

Checklist:

• Code changes
• Unit tests added
• Acceptance tests added
• Documentation ticket raised:

Open tasks:

• Backport (if applicable set "backport-request" label and remove when the backport was done)