It's not needed any more with Go 1.13 as we have wrapping/unwrapping in base
packages. All errors.Wrap calls are replaced with fmt.Errorf, some strings are
improved along the way.
Part of #1055.
It should have `AllowStates` flag.
Also removed unreachable code: we can't have such situation when
script container is not a transaction in the scope of `CheckWitness`
method because:
1. Blocks have their own implementation of CheckWitness for
internal usage (it's (bc *Blockchain) verifyHeaderWitnesses method).
2. For the outside calls of System.Runtime.CheckWitness interop (e.g.
calls from smart-contract) script container is always a transaction.
