DAL/Node: emit error when not able to fetch block in crawler

If there are error while getting a block header in the crawler, probably because the block is too old and the L1 node does not have it, then finalized_heads_stream is simply not filled at all, the daemon cannot process any block, and no error is observed.

This MR emits an event when there's an error, and stops trying to fetch even older blocks. The daemon should thus at least keep processing current blocks.