better error handling [nim-eth-p2p]

[stefantalpalaru]

[this is in relation to https://github.com/status-im/nimbus/issues/198]

• changed the logging level of some messages that should not appear by default in a debug build
• exceptions in persistBlocks() are gracefully recovered from
• fixed the handling of out of order blocks
• dropped work items with fewer blocks than what we requested
• getBestBlockNumber(): log and allow exceptions from getBlockHeaders()
• obtainBlocksFromPeer(): moved here the check for block numbers being in sequence (from nimbus/p2p/chain.nim)
• gracefully deal with exceptions in returnWorkItem()
• sendMsg(): log and allow exceptions, because catching them at the caller's side seems too complicated. I don't see a mechanism for retrying to send these messages.

[stefantalpalaru]

Most exceptions converted to error codes.