Provide feedback to sender when a recipient failed to decrypt a message

Inspiration

When the moderation bot can't decrypt a message sent to him in a room will react to the message with a dedicated emoji 🚨.
This is a precious indicator for the sender as it's the only way for him to know if something went wrong

Proposal

Use the same idea to provide a UI feedback to users when someone did fail to decrypt a message in a room (as a decoration per message or more global indicator).
More than a simple indicator this could be a great place to plug some usefull call to actions:

• Send a bug report
• Re-share key to the sender

Technical input

Such problems could be detected when a key_request is received for the megolm session of this message.

Some difficulties

The weight of the feedback should be different in for example a DM vs a large room (where key request are very likely to happen).

Also the key requests are async and transient:

• You may have the recipient failed to decryptinfo long after sending the message
• The recipient could send a request and then cancel it a few second after because he got the key from another session (we should avoid having a flickering effect by quickly showing hiding feedback).

And as always some filtering should be done to discard evil unexpected requests

Dev task

• Web https://github.com/vector-im/element-web/issues/21906
• iOS
• Android