Registering a right

Just been working my way through the example here and noticed that when creating a right, leaving out the `sourceRightId` causes an application error:

`POST http://localhost:3000/api/v1/rights`

```json
{
"right": {
"license": "My license"
},
"currentHolder": {
"publicKey": "Hrzhm61KQut7Rm2nT1k1f7gGpUWijxsGd8udcPqE5sSK",
"privateKey": "Eiea9RJPRAYqokwVbwr9tw7vt6HWdsZ9TbyFzjaeMdUc"
},
"sourceRightId": ""
}
```

Leaving out any of the other fields results in a 400 with useful information in the body as to why the request was unsuccessful.

Reproduce the POST request to /api/v1/rights with sourceRightId omitted or empty, then inspect the request validation and error handling for right registration. Compare it with validation for the other fields; done means the request returns HTTP 400 with useful error details instead of an application error.

Written by the indexing model from the issue text.