Catching errors when trying to create new TXP

Related to #1245
This PR refactors `Wallet#createTx` to:
  1. Separate its behavior from `Wallet#createPaymentTx` (arguments were given different meaning depending on the context of the call...)
  2. The callback is now passing an `err` argument in addition to the `ntxid` argument.
This last change made it possible to catch errors at tx creation time (e.g. Insight server down).