New onError callback to Meteor.subscribe.
New onError callback to Meteor.subscribe.

This callback receives any terminating error from a subscription. Currently this
can only be because the subscription is unknown, but soon subscriptions will be
able to send other errors.

Also, adjust the semantics of ready callbacks in a reactive context to only call
the latest instance of the callback rather than all callbacks.

Hansoft/meteor,Hansoft/meteor,Hansoft/meteor,Hansoft/meteor,Hansoft/meteor,Hansoft/meteor,Hansoft/meteor