all: consolidate errors

Author: ydnar

errors.go

@@ -1,8 +1,42 @@
 package epp
 
-import (
-	"errors"
-)
+// Error is the interface implemented by all errors in this package.
+type Error interface {
+	eppError()
+}
+
+type stringError string
+
+func (stringError) eppError() {}
+
+func (s stringError) Error() string {
+	return "epp: " + string(s)
+}
+
+// ErrClosedConnection indicates a read or write operation on a closed connection.
+const ErrClosedConnection stringError = "operation on closed connection"
 
 // ErrServerClosed indicates a [Server] has shut down or closed.
-var ErrServerClosed = errors.New("epp: server closed")
+const ErrServerClosed stringError = "server closed"
+
+// TransactionIDError indicates an invalid transaction ID.
+type TransactionIDError struct {
+	TransactionID string
+}
+
+func (TransactionIDError) eppError() {}
+
+// Error implements the error interface.
+func (err TransactionIDError) Error() string {
+	return "epp: invalid transaction transaction ID: " + err.TransactionID
+}
+
+// DuplicateTransactionIDError indicates a duplicate transaction ID.
+type DuplicateTransactionIDError TransactionIDError
+
+func (DuplicateTransactionIDError) eppError() {}
+
+// Error implements the error interface.
+func (err DuplicateTransactionIDError) Error() string {
+	return "epp: duplicate transaction ID: " + err.TransactionID
+}