Add FailedToParse timeline items

[jplatte]

When a timeline event comes in but we fail to deserialize it, previously we would just ignore it. Now, if we can still parse everything except the content, we add an EventTimelineItem with a new kind of content so the fact an event arrived isn't just silently discarded. The raw event and error can be reflected in the UI somehow. If something outside content fails to parse, that is either a bug in the server or in our code and we just log an error.

The first case can easily happen by having a buggy or malicious client, so I think it's important we preserve as much information as possible. In the latter case, just logging an error should be fine as it's much less likely (requires a bug in the server, ruma or the SDK).

[codecov]

Codecov Report

Base: 74.41% // Head: 74.42% // Increases project coverage by +0.00% 🎉

Coverage data is based on head (1b5b3b7) compared to base (43f0ba7).
Patch coverage: 73.73% of modified lines in pull request are covered.

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #1224   +/-   ##
=======================================
  Coverage   74.41%   74.42%           
=======================================
  Files         112      113    +1     
  Lines       12869    12952   +83     
=======================================
+ Hits         9577     9639   +62     
- Misses       3292     3313   +21     

Impacted Files	Coverage Δ
crates/matrix-sdk/src/lib.rs	100.00% <ø> (ø)
crates/matrix-sdk/src/room/timeline/event_item.rs	67.44% <ø> (ø)
crates/matrix-sdk/src/events.rs	70.21% <70.21%> (ø)
...ates/matrix-sdk/src/room/timeline/event_handler.rs	76.39% <76.92%> (+0.55%)	⬆️

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[Hywan]

LGTM, thanks!