Bug 1503: RestconfError "data-missing" is error code 404.

The base specification listed the error code to be 409, which is in
direct violation on HTTP. The restconf specification will be fixed in
the upcoming draft, meanwhile we are going for the new, sane behavior
regardless.

Change-Id: I7a6cf67a794f8348d746ba94b7de9580d0472a61
Signed-off-by: Vratko Polak <vrpolak@cisco.com>
Signed-off-by: Robert Varga <rovarga@cisco.com>
Merge "Bug 1503: RestconfError "data-missing" is error code 404."