message: Use ContextHash format for printing instead of hex encode
diff: @@ -131,7 +131,7 @@ impl ContextApi for TezedgeContext {
  match merkle.get_history(context_hash, &key) {
  Err(MerkleError::ValueNotFound{key: _}) => Ok(None),
  Err(MerkleError::EntryNotFound) => {
  - Err(ContextError::UnknownContextHashError { context_hash: hex::encode(context_hash) })
  + Err(ContextError::UnknownContextHashError { context_hash: HashType::ContextHash.bytes_to_string(context_hash) })
  },
  Err(err) => {
  Err(ContextError::MerkleStorageError { error: err })
  