[storage] get_epoch_change_ledger_infos: better error message
[storage] get_epoch_change_ledger_infos: better error message

Closes: #4511
Approved by: lightmark

libra/libra,aptos-labs/aptos-core,aptos-labs/aptos-core,aptos-labs/aptos-core,libra/libra,libra/libra,aptos-labs/aptos-core,aptos-labs/aptos-core,libra/libra,libra/libra,aptos-labs/aptos-core,aptos-labs/aptos-core