The done_callback for the callback tasks in _run_htlc_switch_iteration
tried to access mpp_sets by key but they might already have been deleted
when the callback is called, causing an KeyError. Instead forward the
exceptions to the crash reporter so we get notice of them and they get
logged correctly.
```
20251219T131356.946565Z | ERROR | asyncio | Exception in callback Peer._run_htlc_switch_iteration.<locals>.<lambda>(<Task finishe.../util.py:1773>) at /home/user/code/electrum-fork/electrum/lnpeer.py:2907
handle: <Handle Peer._run_htlc_switch_iteration.<locals>.<lambda>(<Task finishe.../util.py:1773>) at /home/user/code/electrum-fork/electrum/lnpeer.py:2907 created at /usr/lib64/python3.14/asyncio/events.py:94>
source_traceback: Object created at (most recent call last):
File "/usr/lib64/python3.14/threading.py", line 1082, in _bootstrap_inner
self._context.run(self.run)
File "/home/user/code/electrum-fork/electrum/util.py", line 1145, in run_with_except_hook
run_original(*args2, **kwargs2)
File "/usr/lib64/python3.14/threading.py", line 1024, in run
self._target(*self._args, **self._kwargs)
File "/home/user/code/electrum-fork/electrum/util.py", line 1705, in run_event_loop
loop.run_until_complete(stopping_fut)
File "/usr/lib64/python3.14/asyncio/base_events.py", line 706, in run_until_complete
self.run_forever()
File "/usr/lib64/python3.14/asyncio/base_events.py", line 677, in run_forever
self._run_once()
File "/usr/lib64/python3.14/asyncio/base_events.py", line 2038, in _run_once
handle._run()
File "/usr/lib64/python3.14/asyncio/events.py", line 94, in _run
self._context.run(self._callback, *self._args)
Traceback (most recent call last):
File "/usr/lib64/python3.14/asyncio/events.py", line 94, in _run
self._context.run(self._callback, *self._args)
~~~~~~~~~~~~~~~~~^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
File "/home/user/code/electrum-fork/electrum/lnpeer.py", line 2909, in <lambda>
f"{self.lnworker.received_mpp_htlcs[pk]=}", exc_info=t.exception()) if t.exception() else None
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~^^^^
KeyError: '0000980000010001:1'
```
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