Add a message for outputs being proceessed

[Keith-CY]

This issue comes from nervosnetwork/ckb-explorer#1627

When a transaction is pending, it can be located by transaction hash on the Explorer, but that time its data may be being processed. It confuses users that the transaction has no outputs.

A message should be added to clarify this case.

[Keith-CY]

Should be updated by nervosnetwork/ckb-explorer-frontend@f142459

[FrederLu]

Should be updated by nervosnetwork/ckb-explorer-frontend@f142459

Verified.