fix: exception handler failure when exception is not DBusError

[bdraco]

fixes

Traceback (most recent call last):
  File "src/dbus_fast/aio/message_reader.py", line 26, in dbus_fast.aio.message_reader.build_message_reader._message_reader
  File "src/dbus_fast/message_bus.py", line 807, in dbus_fast.message_bus.BaseMessageBus._process_message
  File "src/dbus_fast/message_bus.py", line 861, in dbus_fast.message_bus.BaseMessageBus._process_message
  File "src/dbus_fast/message_bus.py", line 83, in dbus_fast.message_bus.SendReply.__exit__
  File "src/dbus_fast/message_bus.py", line 64, in dbus_fast.message_bus.SendReply._exit
AttributeError: type object Exception has no attribute _as_message

[codecov]

Codecov Report

Patch coverage: 88.88% and project coverage change: +0.11% 🎉

Comparison is base (5bda04b) 82.47% compared to head (aa18c42) 82.59%.
Report is 1 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #215      +/-   ##
==========================================
+ Coverage   82.47%   82.59%   +0.11%     
==========================================
  Files          27       27              
  Lines        3212     3211       -1     
  Branches      666      665       -1     
==========================================
+ Hits         2649     2652       +3     
+ Misses        345      343       -2     
+ Partials      218      216       -2     

Files Changed	Coverage Δ
src/dbus_fast/__version__.py	0.00% <0.00%> (ø)
src/dbus_fast/message_bus.py	72.34% <100.00%> (+0.49%)	⬆️

... and 1 file with indirect coverage changes

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[bdraco]

manual testing looks good