New test for another error case in DepositScriptAddress

The comment just states that we mock cryptoKeyScript.Encrypt() and that
we expect the error returned by our mock to be propagated by
DepositScriptAddress. I'd keep the comment regardless of whether or not
I used a callback.

Closures are used frequently in most Go projects I've seen so far, and
I've never seen them described as an "advanced CS concept". As for the
callback, I think you're confusing that with callback-based async
programming (e.g. what Twisted offers for Python), which is indeed
complex and error prone (partly because of the inherent complexities of
async programming, though). But having a function which takes a callback
and calls it synchronously is a pretty standard thing and nowhere near
an advanced CS concept.