Add test for DisconnectPeer event
Update DisconnectPeer with optional ErrorMessage
Manage error for funding_transaction_generated
Add disconnect_socket to SocketDescriptor trait
Summary:
Implementing these, taking inspiration from the surrounding code since i'm a rust n00b.
Test Plan:
```
cargo build
cargo test channel_reestablish
```
output:
```
running 3 tests
test ln::msgs::tests::encoding_channel_reestablish_with_secret ... ok
test ln::msgs::tests::decode_channel_reestablish_bad_length ... ok
test ln::msgs::tests::encoding_channel_reestablish_no_secret ... ok
test result: ok. 3 passed; 0 failed; 0 ignored; 0 measured; 14 filtered out
```
this was a TODO and also briefly discussed in https://github.com/rust-bitcoin/rust-lightning/pull/43#pullrequestreview-135649551
I'm not fully sure how to remove the `Option`, and make it completely required. Would love suggestions. So, have omitted that for now. Plus, better to make smaller, incremental changes.
Test Plan:
`cargo build`
`cargo test`
It obviously was nonsensical prior, though its now way more
complicated...there's refactoring work to be done here, but it
should at least kinda sorta work now.
