BUG 1144 - response 500 instead of 400

RestconfDocumentedException with error tag INVALID_VALUE (400) was caught and
wrapped to RestconfDocumentedException with error tag OPERATION_FAILED
(500)

Change-Id: I0ad75958e0d8e25d5bf1bdf7cbac5f855cc2b03d
Signed-off-by: Jozef Gloncak <jgloncak@cisco.com>