Show a more helpful error when trying to add an event on ReadOnlyReplica

[hayley-jean]

ReadOnlyReplica nodes are not able to accept or forward writes, and at the moment they return an internal server error if you try and append an event through the UI.
This error is unhelpful and should be improved to explain why the write failed.

[hayley-jean]

We need to decide if Read Only Replicas should forward writes to the Leader, or if they should continue to reject writes.