Removed unreachable code, see
8fed383523
runtime.CheckHashedWitness can only be used for transaction
verification, the other two options from reference implementation (block
and consensus payload) have separate methods for verification.
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.
