Fixed issue with not attaching errors when propertyPaths could not be resolved

[acizmarik]

This PR is cherry-picking important bugfix from #1498. Right now, when attaching errors on an non-existing object, the function throws. Therefore, we never trigger the validationErrorsChanged event. At the end even valid errors are not displayed.

Users can encounter this issue in the following ways:

• Manually specifying incorrect property paths using our string notation
• Providing correct expression or correct property path but the viewmodel changes (i.e. the property where we need to attach errors is no longer reachable from root viewmodel)