Update RESTCONF error mapping

Update RESTCONF error mapping

yang.common.YangError is an attachment used to propagate RFC6020-defined
failure modes enforced by YANG Tools as exceptions. One such example is
YangInvalidValueException, which is reporting YANG constraint
violations.

Recognize these when mapping to RestconfDocumentedException, so that
their error-app-tag and error-message are properly propagated.

JIRA: NETCONF-786
Change-Id: Id6ea1877e03f42fc193942a4e2b83ecb6daeb631
Signed-off-by: Robert Varga <robert.varga@pantheon.tech>