Funds checks in place, so adding user-facing message if low on funds

You'll have to tell me how useful this is... 

Since the funds checks are already in place for trade_MP, I decided to test out how a user-facing message for low user funds would look (I think it looks better than 'error code=-1'). 

We could conceivably fill this with the rest of the error codes and across the full RPC, depending on how this initial change looks to everyone.

Example response:
```
./mastercored trade_MP  mfaiZGBkY4mBqt3PHPD2qWgbaafGa7vR64 10000000000 1 2000000000 3 1
error: {"code":-8,"message":"error_code= -1, not enough funds in user address "}
```