Error when sending event are not correctly handled

When sending an Event, if an error 400 occurs, the user is not informed of the failure. The Event stays in grey in the timeline forever.

To simulate an error 400, we can hack the code and send a listOf(Content) instead of Content when calling [RoomAPI.send()](https://github.com/vector-im/element-android/blob/develop/matrix-sdk-android/src/main/java/org/matrix/android/sdk/internal/session/room/RoomAPI.kt#L125)