Return signer failure to `ChannelManager` to stop accepting new channels

[ariard]

See #2089 (comment).

[TheBlueMatt]

This will be implied by any fully fallible signer interface - if we can't get to the signer we can't get keys for a new channel or sign the initial commitment transaction.

[ariard]

This will be implied by any fully fallible signer interface - if we can't get to the signer we can't get keys for a new channel or sign the initial commitment transaction.

Unless you're signer interface is not fully fallible and you have partial signer failing (e.g due to UTXO oracle being out or disconnection from a partial signer) and not keys generation.

I just don't buy the "fully fallible" (all or nothing), if you have a component failing better to handle back the error to LDK, stop processing new channels, while still being able to sign time-sensitive channel if you can.

[TheBlueMatt]

I don't really get what component would fail in a signer without the full signer failing? If the signer isn't plugged in, everything fails, if the signer has a hardware failure, its not going to start failing on some functions but not on others? Its possible we can build pubkeys for a signer without being able to sign, but accepting new channels isn't really the biggest issue, we can't enforce anything without our signer, so we should fix that (have the user plug it in/whatever) not worry about just some subset (the new ones) of our channels.