box not found in state: error message must print the box id correctly

[paolocappelletti]

please fix this error message:
https://github.com/HorizenOfficial/Sidechains-SDK/blob/master/sdk/src/main/scala/com/horizen/SidechainState.scala#L281

Actually the box id is not printed correctly, making more difficult the debug:
{"code":"0204","description":"GenericTransactionError","detail":"Box [B@3180d476 is not found in state"}

[i-Alex]

need to define toString method properly for key classes