[#147] Add constant error type

closes #147

Signed-off-by: Evgeniy Kulikov <kim@nspcc.ru>

internal/error.go

@@ -0,0 +1,9 @@
+package internal
+
+// Error is constant type error.
+type Error string
+
+// Error implementation of error interface.
+func (e Error) Error() string {
+	return string(e)
+}