Better error message when sending to both sprout and sapling
When trying to send to both Sprout and Sapling (not currently supported with z_sendmany) we were getting the following error in our operation result: `general exception: boost::bad_get: failed value get using boost::get`.
This PR changes this to fail with a better error message before the async operation begins:
```
error code: -8
error message:
Cannot send to both Sprout and Sapling addresses using z_sendmany
```
527b7feef5