[Private transactions] Returning informative private transaction status info/error instead of silent failiure

[ngyam]

• Parity Ethereum version: Parity-Ethereum/v2.2.0-unstable-93e1040-20180921/x86_64-linux-gnu/rustc1.29.0
• Operating system: Linux
• Installation: built from source with secretstore feature
• Fully synchronized: yes
• Network: Tobalaba/dev/local PoA
• Restarted: yes

actual

When sending a private transaction with private_sendTransaction, e.g. modifying a value as in the Parity private transactions tutorial, a similar response is returned:

{ contractAddress: '0x5f72cc768e8de3d78e071c05a9a41a44e88a779a',
  status: 0,
  transactionHash: '0x1c3ae1a8897fc3c1da6e442006c8d35a77e8518803f4604c21c6653a8ba631a9' }

This return message was not found to be helpful, because:

1. In case some error happens, the transaction fails silently (e.g. some error with secret store). There is no way to figure out what happened without digging the logs.
2. There is no way to figure out if your transaction is ever gonna happen and when is it gonna happen. Should I wait some more? Retry? How can I track the private transaction status?

This is a limitation for a production grade application.

expected behavior

Is it possible to improve user experience with:

• returning an error message in case of a failiure
• provide a means of tracking the status of the private transaction. Is it still being verified by the private validators? Did it get rejected? Reached the transaction pool already? etc.

steps to reproduce

I made a repo to easily play around with secretstore/private transactions: https://github.com/ngyam/tutorial-secretstore-privatetx

I couldn't make private transactions work on live Tobalaba (start nodes in nodes_ss_tobalaba/ folder), so that one is worth to check out. Modifying the value of x as in tutorial part 4 doesn't work.