State events that fail to deserialize should not be ignored

[zecakeh]

This is a problem similar to #607: state events that fail to deserialize are ignored.

But usually in the spec a state event that has invalid fields in the content still replaces the previous state event. It is just treated as if there was no state event of that type to begin with.

I believe that's due to servers not having to validate the content of state events outside of state res, so they always treat any accepted state event as the current state.

Examples from the spec:

• For m.space.child:

  Child rooms can be removed from a space by omitting the via key of content on the relevant state event, such as through redaction or otherwise clearing the content.

  Although thevia field is now marked as required: Clarify that via is required for m.space.parent and m.space.child as per MSC1772. matrix-spec#1618.

• For m.room.name:

  A room with an m.room.name event with an absent, null, or empty name field should be treated the same as a room with no m.room.name event.