output error msg

[realcodywburns]

Failure to write the output or joinsplit returns the same error as failure to write spend

File:compute.rs L32-34:

sapling_spend.write(&mut new_params).expect("couldn't write new Sapling Spend params"); 
sapling_output.write(&mut new_params).expect("couldn't write new Sapling Spend params"); 
sprout_joinsplit.write(&mut new_params).expect("couldn't write new Sapling Spend params");