Consider ignoring errors from the exchange in the blockservice

[Stebalien]

Currently, the blockservice can succeed in adding a block and then fail because it fails somewhere when telling the exchange we have the block.

We should probably just log and ignore (and, maybe, change the HasBlock signature on the exchange to not return an error).

[cannium]

I happen to encounter the comments when reading related code. Some thoughts:

• exchange.HasBlock return error only if the write to datastore fails. If it fails, would peers know the new blocks?(haven't read about bitswap...) If not, then we should not simply ignore the error, at least make it optional for users to choose.
• Would some kind of batch processing be useful here, like exchange.HasBlocks ?