Implement notifications service with shared messaging and email drivers (#36)

backend/shared/messaging/capability/errors.go

@@ -0,0 +1,19 @@
+package capability
+
+import (
+	"strings"
+
+	"git.sendico.io/tech/sxchnge/shared/errors"
+)
+
+var ErrCapabilityUnavailable = merrors.Define("capability unavailable")
+
+var ErrClosed = merrors.Define("closed")
+
+func Unavailable(name string) error {
+	name = strings.TrimSpace(name)
+	if name == "" {
+		return ErrCapabilityUnavailable
+	}
+	return merrors.Wrap(ErrCapabilityUnavailable, name)
+}