Cleanup ModificationApplyOperation.verifyStructure confusion

ModificationApplyOperation.verifyStructure() has a slightly unclear contract and has an unclear relationship with ModificationApplyOperation.recursivelyVerifyStructure().

There are two things that need to happen:

• verifyChildren=true needs to be split into a new method, which will throw a checked exception
• recursivelyVerifyStructure() needs to have its contract documented

For reasoning see:

• AbstractNodeContainerModificationStrategy implementation of both methods – which share majority of the code
• MinMaxElementsValidation and MandatoryLeafEnforcer.enforceOnData() callers, who could use the ability to report a DataValidationFailedException (and requisite context where we are in the data tree)