Bug 3822: Improve error reporting for restconf PUT

Bug 3822: Improve error reporting for restconf PUT

A runtime exception can be emitted by the netconf mount point which
should be reported to the user, otherwise you get a 500 response with
no error info which isn't very helpful.

Also the fucntionality to output the error-info field was ommitted with
the conversion from CompositeNode to NormalizedNode so I re-implemeneted
it. It was originally ommitted with a TODO b/c the
NormalizedNodeStreamWriters validate against the schema and error-info
is defined as an empty container in the restconf yang. So there's no way
to create a ContainerNode to represent the error-info data that conforms
to the schema. To work around this, I created a leaf node and special-cased
error-info in the stream writer to elide schema validation.

I also added a regression unit test for the case where the URL contains
an identityref.

Change-Id: I4bb0d767bb8008023e7ef10a439025e2e591f9cd
Signed-off-by: Tom Pantelis <tpanteli@brocade.com>