Use descriptive URIs in 104 Warning messages

This makes the new GPG related warnings much nicer to read,
for example, the second one here replaces the first one:

W: gpgv:/var/lib/apt/lists/example.com_dists_stable_InRelease: Weak ...
W: http://example.com/dists/stable/InRelease: Weak ...

apt-pkg/acquire-worker.cc

@@ -279,7 +279,7 @@ bool pkgAcquire::Worker::RunMessages()
          }
 	 // 104 Warning
 	 case 104:
-	    _error->Warning("%s: %s", Itm->URI.c_str(), LookupTag(Message,"Message").c_str());
+	    _error->Warning("%s: %s", Itm->Owner->DescURI().c_str(), LookupTag(Message,"Message").c_str());
 	    break;
 
 	 // 200 URI Start