These will need to handle `Meteor.Error`s as well as normal `Error`; that is, when we stick a `Meteor.Error` into this collection, it should be read out as `Meteor.Error` so that it will get sent down to the client.

It's okay if other error classes get reconstructed as plain `Error`s because livedata wraps them as "500 Internal server error" anyway (see `wrapInternalException` in livedata_server.js), but it would be great to have a comment noting this.