admin: Respond with 4xx on non-existing config path (#5870)

Co-authored-by: Matt Holt <mholt@users.noreply.github.com>

admin_test.go

@@ -75,6 +75,12 @@ func TestUnsyncedConfigAccess(t *testing.T) {
 			path:   "/bar/qq",
 			expect: `{"foo": "jet", "bar": {"aa": "bb"}, "list": ["a", "b", "c"]}`,
 		},
+		{
+			method:    "DELETE",
+			path:      "/bar/qq",
+			expect:    `{"foo": "jet", "bar": {"aa": "bb"}, "list": ["a", "b", "c"]}`,
+			shouldErr: true,
+		},
 		{
 			method:  "POST",
 			path:    "/list",