MODE-2005 Fixed the setting of the jcr:mergeFailed property in case of versionable node conflicts

The problem did not only occur when a merge failed during the `merge()` call, but also if `cancelMerge` or `doneMerge` were called.

It was caused by JCR-layer property validations which should not take place in the above cases for the `jcr:mergeFailed` property.