lntransport: CancelledError needs priority over LPConnClosed

Scenario (prior this change):
A task in lnpeer.Peer.taskgroup raises ORIG_EXC, e.g. in htlc_switch.
The taskgroup then cancels all its tasks and then awaits each (in cancel_remaining):
4e64c56042/aiorpcx/curio.py (L217-L221)
In Peer.main_loop, we would want ORIG_EXC to be raised,
but instead LightningPeerConnectionClosed() will be raised as
the Peer._message_loop() task is cancelled, and it is awaited first in cancel_remaining.
We should make sure that if a task is cancelled it will let the CancelledError
propagate out, or at least it does not raise a different exception instead.

electrum/lntransport.py

@@ -123,7 +123,9 @@ class LNTransportBase:
                         break
                 try:
                     s = await self.reader.read(2**10)
-                except:
+                except asyncio.CancelledError:
+                    raise
+                except Exception:
                     s = None
                 if not s:
                     raise LightningPeerConnectionClosed()