Expose event sending errors through timeline item

[jplatte]

… instead of through the return value of Timeline::send.

I think this should be easier to work with for users of the timeline API. cc @zecakeh

[codecov]

Codecov Report

Base: 72.93% // Head: 72.95% // Increases project coverage by +0.01% 🎉

Coverage data is based on head (fcfa697) compared to base (e9cef35).
Patch coverage: 69.23% of modified lines in pull request are covered.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #1423      +/-   ##
==========================================
+ Coverage   72.93%   72.95%   +0.01%     
==========================================
  Files         120      120              
  Lines       13137    13139       +2     
==========================================
+ Hits         9581     9585       +4     
+ Misses       3556     3554       -2     

Impacted Files	Coverage Δ
crates/matrix-sdk/src/room/timeline/event_item.rs	49.11% <60.00%> (+0.91%)	⬆️
crates/matrix-sdk/src/room/timeline/inner.rs	74.22% <70.00%> (+0.99%)	⬆️
crates/matrix-sdk/src/room/timeline/mod.rs	82.44% <80.00%> (-0.24%)	⬇️
...ates/matrix-sdk/src/room/timeline/event_handler.rs	68.63% <100.00%> (-0.09%)	⬇️

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[zecakeh]

Looks fine to me.

[Hywan]

Thanks for improving my PR :-).