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 "}
```
Well, where does -1 come from :) Kinda of a rhetorical.
Since we are starting this all potential user-facing error codes should be done similar to:
// Bitcoin RPC error codes
enum RPCErrorCode
{
    // Standard JSON-RPC 2.0 errors
    RPC_INVALID_REQUEST  = -32600,
    RPC_METHOD_NOT_FOUND = -32601,
    RPC_INVALID_PARAMS   = -32602,
    RPC_INTERNAL_ERROR   = -32603,
    RPC_PARSE_ERROR      = -32700,

    // General application defined errors
    RPC_MISC_ERROR                  = -1,  // std::exception thrown in command handling
    RPC_FORBIDDEN_BY_SAFE_MODE      = -2,  // Server is in safe mode, and command is not allowed in safe mode
    RPC_TYPE_ERROR                  = -3,  // Unexpected type was passed as parameter
    RPC_INVALID_ADDRESS_OR_KEY      = -5,  // Invalid address or key
    RPC_OUT_OF_MEMORY               = -7,  // Ran out of memory during operation

etc.