Fix wrong usage of ErrInvalidTypeAssertion

A few calls of ErrInvalidTypeAssertion where not right and resulted in a
meaningless error message output.

nucleus/southbound.go

@@ -2,7 +2,6 @@ package nucleus
 
 import (
 	"path/filepath"
-	"reflect"
 
 	"code.fbi.h-da.de/danet/gosdn/nucleus/errors"
 
@@ -249,8 +248,8 @@ func (p *SouthboundPlugin) load(id uuid.UUID) error {
 	if !ok {
 		p.state = plugin.FAULTY
 		return &errors.ErrInvalidTypeAssertion{
-			Value: reflect.TypeOf(symbol),
-			Type:  reflect.TypeOf((*southbound.SouthboundInterface)(nil)).Elem(),
+			Value: symbol,
+			Type:  (*southbound.SouthboundInterface)(nil),
 		}
 	}
 	// Note(mbauch): We could consider moving this into plugin creation.

store/deviceStore.go

@@ -81,7 +81,7 @@ func (s *DeviceStore) GetDevicesAssociatedWithSbi(sid uuid.UUID) ([]device.Devic
 		if !ok {
 			return nil, &errors.ErrInvalidTypeAssertion{
 				Value: d,
-				Type:  reflect.TypeOf((*device.Device)(nil)),
+				Type:  (*device.Device)(nil),
 			}
 		}
 		// check if the device uses the provided SBI and add it to the devices

store/sbiStore.go

@@ -60,8 +60,8 @@ func (s *SbiStore) Add(item store.Storable) error {
 	_, ok := item.(southbound.SouthboundInterface)
 	if !ok {
 		return &errors.ErrInvalidTypeAssertion{
-			Value: reflect.TypeOf(item),
-			Type:  reflect.TypeOf((*southbound.SouthboundInterface)(nil)).Elem(),
+			Value: item,
+			Type:  (*southbound.SouthboundInterface)(nil),
 		}
 	}
 
@@ -96,16 +96,16 @@ func (s *SbiStore) persist() error {
 		southboundInterface, ok := value.(southbound.SouthboundInterface)
 		if !ok {
 			return &errors.ErrInvalidTypeAssertion{
-				Value: reflect.TypeOf(value),
-				Type:  reflect.TypeOf((*southbound.SouthboundInterface)(nil)).Elem(),
+				Value: value,
+				Type:  (*southbound.SouthboundInterface)(nil),
 			}
 		}
 		if southboundInterface.Type() == spb.Type_TYPE_CONTAINERISED || southboundInterface.Type() == spb.Type_TYPE_PLUGIN {
 			southboundPlugin, ok := southboundInterface.(plugin.Plugin)
 			if !ok {
 				return &errors.ErrInvalidTypeAssertion{
-					Value: reflect.TypeOf(southboundInterface),
-					Type:  reflect.TypeOf((*plugin.Plugin)(nil)).Elem(),
+					Value: southboundInterface,
+					Type:  (*plugin.Plugin)(nil),
 				}
 			}
 			southboundInterfacesToPersist = append(southboundInterfacesToPersist, LoadedSbi{