Begun data module

core/data/errors.go

@@ -0,0 +1,10 @@
+package data
+
+import "errors"
+
+var (
+	ErrUnknownKey        = errors.New("unknown key")
+	ErrAlreadySubscribed = errors.New("already subscribed")
+	ErrNotSubscribed     = errors.New("not subscribed")
+	ErrDroppedMessage    = errors.New("dropped message: buffer full")
+)