shim: proper error handling

[pepyakin]

specifically, a few unwraps scattered around the codebase. Specifically, if you sovereign_getBlock with node-url set to a non-sugondat node, it will panic with the error

thread 'tokio-runtime-worker' panicked at 'called Result::unwrap() on an Err value: no tree root found in block header. Are you sure this is a sugondat node?', sugondat-shim/src/adapters/sovereign.rs:27:64
note: run with RUST_BACKTRACE=1 environment variable to display a backtrace

(Another issue is that sovereign seems to not receive the memo that it panicked, so it kept waiting)

[rphmeier]

Yes - we should expunge as many unwrap as possible.

I really like the policy from Parity where we always use expect and provide a string arguing the case for impossibility.