Update mattermost library

vendor/github.com/wiggin77/merror/append.go

@@ -0,0 +1,25 @@
+package merror
+
+// Append an error to a multi-error.
+// If `to` is `nil` it will just assign `err`.
+// If `to` is not a `*MError` it will create a new `*MError` and append both errors.
+// If `err` is `nil` it will just return `to`.
+// Otherwise it will just append to the existing `*MError`.
+func Append(to, err error) error {
+	if err == nil {
+		return to
+	}
+	if to == nil {
+		return err
+	}
+
+	if merr, ok := to.(*MError); ok {
+		merr.Append(err)
+		return merr
+	}
+
+	merr := New()
+	merr.Append(to)
+	merr.Append(err)
+	return merr
+}