Constant errors

errors.go

@@ -0,0 +1,10 @@
+package melody
+
+import "errors"
+
+var (
+	ErrClosed            = errors.New("melody instance is closed")
+	ErrSessionClosed     = errors.New("session is closed")
+	ErrWriteClosed       = errors.New("tried to write to closed a session")
+	ErrMessageBufferFull = errors.New("session message buffer is full")
+)