Fix exception handling in PatchOp's replace operation

[sleelin]

Previously, PatchOp's internal replace operation handling method would rethrow any exceptions thrown when attempting to remove a value before re-adding it, provided it didn't have a scimType of noTarget. This meant that required attribute values could not be replaced, as the call to remove the required attribute's value threw an exception with a scimType of invalidValue, despite having the value immediately added back.

Ultimately, all exceptions thrown in the removal phase of the replace operation can safely be discarded, as the subsequent call to re-add the value will perform its own constraint validation, thereby throwing the same relevant exception thrown in the removal phase (fixes #16).

The test fixtures for the PatchOp message class have also been updated to check that replace operations for required attributes behaves as expected.