[#218] pkg/object: Define custom error for SplitInfo

Signed-off-by: Alex Vanin <alexey@nspcc.ru>

pkg/object/error.go

@@ -0,0 +1,19 @@
+package object
+
+type SplitInfoError struct {
+	si *SplitInfo
+}
+
+const splitInfoErrorMsg = "object not found, split info has been provided"
+
+func (s SplitInfoError) Error() string {
+	return splitInfoErrorMsg
+}
+
+func (s SplitInfoError) SplitInfo() *SplitInfo {
+	return s.si
+}
+
+func NewSplitInfoError(v *SplitInfo) SplitInfoError {
+	return SplitInfoError{si: v}
+}