message: Improved error message
  Summary: Error message was confusing one of our users. We thought a case happened only once, but actually could happen for another reason as well.
  Reviewers: alex
  Subscribers: ben
diff: @@ -493,10 +493,10 @@ exports.createTransaction = function(params) {
  var err;
  if (totalUnspentsCount === fetchedUnspentsCount) {
  // we fetched every unspent the wallet had, but it still wasn't enough
  - err = new Error('Insufficient funds');
  + err = new Error('Insufficient funds. Can send only ' + inputAmount + ' satoshis.');
  } else {
  // we weren't able to fetch all the unspents on the wallet
  - err = new Error('Transaction size too large due to too many unspents. Can send only ' + inputAmount + ' satoshis in this transaction');
  + err = new Error('Insufficient spendable funds. Can send only ' + inputAmount + ' satoshis in a single transaction. There are either pending incoming transactions, or the transaction size limit was reached.');
  }
  err.result = {
  fee: fee,
  