Format error by value

- Use `%v` verb to format errors.
- Give `param` constant in portallocator some better name.

Signed-off-by: Michal Minar <miminar@redhat.com>

daemon/graphdriver/graphtest/graphtest.go

@@ -73,7 +73,7 @@ func newDriver(t *testing.T, name string) *Driver {
 
 	d, err := graphdriver.GetDriver(name, root, nil)
 	if err != nil {
-		t.Logf("graphdriver: %s\n", err.Error())
+		t.Logf("graphdriver: %v\n", err)
 		if err == graphdriver.ErrNotSupported || err == graphdriver.ErrPrerequisites || err == graphdriver.ErrIncompatibleFS {
 			t.Skipf("Driver %s not supported", name)
 		}