Wallet: better error for send() if no outputs given

[pinheadmz]

If you try to wallet.send() with an empty outputs array, the error you currently get is:
No outputs available.

This was confusing me because I thought the wallet was unable to find any coins to spend, like it had a zero balance or coinbase outputs were not mature yet.

This PR changes that error to At least one output required. which indicates to the user/developer more specifically which output is missing.

[tynes]

Good change. I agree with this

[BluSyn]

👍

[braydonf]

Okay, that is more clear.

While testing, also got this message as a result of a missing address (slightly incorrect):

./bin/bwallet-cli --network=regtest send 
Error: Output is dust.
    at WalletClient.request (/home/bgf/c/bcoin/bcoin/node_modules/bcurl/lib/client.js:213:19)
    at process._tickCallback (internal/process/next_tick.js:68:7)

It also looks like lib/wallet/wallet isn't used in any tests, only lib/wallet/walletdb.

Here is a test case after adding const Wallet = require('../lib/wallet/wallet'); to test/wallet-test.js:

it('should throw error with missing outputs', async () => {
  const wallet = new Wallet({});

  let err = null;

  try {
     await wallet.send({outputs: []});
  } catch (e) {
    err = e;  
 }

  assert(err);
  assert.equal(err.message, 'At least one output required.')
});

[braydonf]

nit: commit message is without wallet: prefix

[pinheadmz]

nit addressed and test added. Re: output is dust, I think that makes sense, the first thing it's checking after the basic shape of the input object is the value of the first output, which in your example is null and probably cast to 0