Fix lite client crashing when block lookup error

[hey-researcher]

Here is a line: https://github.com/ton-blockchain/ton/blob/037053fffec58dedf8e72051fb695a4a42d3d9f3/tonlib/tonlib/TonlibClient.cpp#L5567C60-L5567C77

If the liteserver did not succeed in looking up the block, .ok() will perform a check and crash the app.

[jamescarter-le]

Any chance of getting this PR merged or someone from TON taking a look at this, as causing crashes apps consuming this library.

[hey-researcher]

@EmelyanenkoK could check it please