Error reporting tests

Has a failing test, showing that the presence of the `Amount` field isn't checked in a Payment

```
  1) errorReporting Signing test (6): Missing field 'tx_json.Amount' :
     Uncaught AssertionError: Expected `Missing field 'tx_json.Amount'` in
 {
  "error": "invalidTransaction",
  "error_exception": "transaction not valid",
  "request": {
    "command": "sign",
    "secret": "masterpassphrase",
    "tx_json": {
      "Account": "rHb9CJAWyB4rj91VRWn96DkukG4bwdtyTh",
      "Destination": "rMTzGg7nPPEMJthjgEBfiPZGoAM7MEVa1r",
      "TransactionType": "Payment"
    }
  },
  "status": "error"
}
```