Fix lack of error passing for nested preload

[michalg9]

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• Do only one thing
• No API-breaking changes
• New code/logic commented & tested

For significant changes like big bug fixes, new features, please open an issue to make an agreement on an implementation design/plan first before starting it.

What did this pull request do?

For nested preloads there seems to be no passing of errors if a preload query fails. So for example in a test case from this PR, if Level2 preload fails with error for err = localDB.Preload("Level2").Find(&got).Error, then err != nil.

However, for nested preload fails this is not the case. If Level1 preload will fail with error from the DB, for the err = localDB.Preload("Level2.Level1").Find(&got).Error query we will have err == nil. This may lead to segmentation faults when objects that are expected to be preloaded (because err == nil) are in fact not preloaded and empty.

This PR fixes that by checking currentScope errors on preload callbacks.