channeld: exit after we send an error at lightningd's request.
Otherwise connectd complains we didn't close, eg
tests/test_connection.py::test_funding_cancel_race:
```
lightningd-1 2022-07-17T14:43:56.813Z DEBUG 022d223620a359a47ff7f7ac447c85c46c923da53389221a0054c11c1e3ca31d59-channeld-chan#1: Send error reason: Cancel channel by our RPC command before funding transaction broadcast.
lightningd-1 2022-07-17T14:43:56.867Z DEBUG 022d223620a359a47ff7f7ac447c85c46c923da53389221a0054c11c1e3ca31d59-channeld-chan#1: peer_out WIRE_ERROR
lightningd-2 2022-07-17T14:43:56.926Z DEBUG 0266e4598d1d3c415f572a8488830b60f7e744ed9235eb0b1ba93283b315c03518-channeld-chan#1: peer_in WIRE_ERROR
lightningd-2 2022-07-17T14:43:56.951Z DEBUG 0266e4598d1d3c415f572a8488830b60f7e744ed9235eb0b1ba93283b315c03518-channeld-chan#1: billboard perm: Received error channel d11396cbb8de10f02ee8d76ff6265bad0eefa7e43b4f540f14dfaab851aa3606: Cancel channel by our RPC command before funding transaction broadcast.
lightningd-1 2022-07-17T14:43:56.952Z DEBUG 022d223620a359a47ff7f7ac447c85c46c923da53389221a0054c11c1e3ca31d59-channeld-chan#1: Status closed, but not exited. Killing
lightningd-2 2022-07-17T14:43:56.976Z UNUSUAL 0266e4598d1d3c415f572a8488830b60f7e744ed9235eb0b1ba93283b315c03518-chan#1: Peer permanent failure in CHANNELD_AWAITING_LOCKIN: channeld: received ERROR error channel d11396cbb8de10f02ee8d76ff6265bad0eefa7e43b4f540f14dfaab851aa3606: Cancel channel by our RPC command before funding transaction broadcast., forget channel
DEBUG:root:Received response for fundchannel_cancel call: {'jsonrpc': '2.0', 'id': 1, 'result': {'cancelled': 'Channel open canceled by RPC(after fundchannel_complete)'}}
DEBUG:root:Received response for fundchannel_cancel call: {'jsonrpc': '2.0', 'id': 1, 'result': {'cancelled': 'Channel open canceled by RPC(after fundchannel_complete)'}}
DEBUG:root:{
"id": 1,
"result": {
"cancelled": "Channel open canceled by RPC(after fundchannel_complete)"
}
}
DEBUG:root:{
"id": 1,
"result": {
"cancelled": "Channel open canceled by RPC(after fundchannel_complete)"
}
}
DEBUG:root:{
"id": 1,
"method": "txdiscard",
"params": {
"txid": "0736aa51b8aadf140f544f3be4a7ef0ead5b26f66fd7e82ef010deb8cb9613d1"
}
}
lightningd-1 2022-07-17T14:43:57.022Z DEBUG connectd: drain_peer
DEBUG:root:Calling txdiscard with payload {'txid': '0736aa51b8aadf140f544f3be4a7ef0ead5b26f66fd7e82ef010deb8cb9613d1'}
lightningd-1 2022-07-17T14:43:57.024Z DEBUG connectd: drain_peer draining subd!
lightningd-1 2022-07-17T14:43:57.069Z DEBUG 022d223620a359a47ff7f7ac447c85c46c923da53389221a0054c11c1e3ca31d59-lightningd: peer_disconnect_done
lightningd-1 2022-07-17T14:43:57.082Z DEBUG 022d223620a359a47ff7f7ac447c85c46c923da53389221a0054c11c1e3ca31d59-lightningd: Not reconnecting: no active channel
...
lightningd-1 2022-07-17T14:44:01.877Z **BROKEN** 022d223620a359a47ff7f7ac447c85c46c923da53389221a0054c11c1e3ca31d59-connectd: Subd did not close, forcing close
```
Signed-off-by: Rusty Russell <rusty@rustcorp.com.au>
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