Changes to handle invalid coin(s) in privacy-wallet-service.

[snehal-das]

• Problem Overview
  Current privacy library does not handle invalid coins gracefully. The logic of claiming coins is modified to handle this scenario.

• Testing Done
  Testcase: Create a scenario where an invalid coin is transferred to a user. Observe how the invalid coin is handled at the recipient's end. Observation: The invalid coin is no longer reported to the DApp, the user does not notice any issue and future transactions are synced successfully.

[snehal-das]

Changes to the privacy-wallet-service is complete and tested to work as expected. Changing status from draft to ready for merge.

[snehal-das]

I am not able to edit the reviewers list, the only option I see is to mention the person here.

@bsenthilr, @yontyon Please take a look and add your thoughts.

Some items pending are:

1. Add a proper error response instead of sending back an error string.
2. Correct the initialisation of invalidCoinsInTransfer.
3. Look for better options for error reporting.