ContactLoader: set requested URI to error/not found result

- Now both the "error" and "not found" results have the requested URI.
- NOT_FOUND is no longer a singleton; there's now "isNotFound()" method
in accordance with isError().

Bug 5350017
Bug 5201346

Change-Id: Icb9080d3885ea2217e73a59db3825984746c3eb3