fix(esplora): `chain_update` errors if no point of connection

fix(esplora): `chain_update` errors if no point of connection

Before, the `chain_update` function would hit a panic if the
local checkpoint was not on the same network as the remote
server. Now if we have iterated all of the blocks of the
`local_cp` and do not find a "point of agreement", then we
return early with a `esplora_client::Error::HeaderHashNotFound`.