CopyFail should be frontend message

backend.go

@@ -14,6 +14,7 @@ type Backend struct {
 	// Frontend message flyweights
 	bind            Bind
 	_close          Close
+	copyFail        CopyFail
 	describe        Describe
 	execute         Execute
 	flush           Flush
@@ -80,6 +81,8 @@ func (b *Backend) Receive() (FrontendMessage, error) {
 		msg = &b.describe
 	case 'E':
 		msg = &b.execute
+	case 'f':
+		msg = &b.copyFail
 	case 'H':
 		msg = &b.flush
 	case 'P':

copy_fail.go

@@ -11,7 +11,7 @@ type CopyFail struct {
 	Message string
 }
 
-func (*CopyFail) Backend() {}
+func (*CopyFail) Frontend() {}
 
 func (dst *CopyFail) Decode(src []byte) error {
 	idx := bytes.IndexByte(src, 0)

frontend.go

@@ -22,7 +22,6 @@ type Frontend struct {
 	copyInResponse       CopyInResponse
 	copyOutResponse      CopyOutResponse
 	copyDone             CopyDone
-	copyFail             CopyFail
 	dataRow              DataRow
 	emptyQueryResponse   EmptyQueryResponse
 	errorResponse        ErrorResponse
@@ -82,8 +81,6 @@ func (b *Frontend) Receive() (BackendMessage, error) {
 		msg = &b.dataRow
 	case 'E':
 		msg = &b.errorResponse
-	case 'f':
-		msg = &b.copyFail
 	case 'G':
 		msg = &b.copyInResponse
 	case 'H':