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This is a pull request for issues #455 and #456.

Hm, it seems to me that what they are saying on Stack Overflow is this:

If we have
```
class FormError extends Meteor.Error
err = new FormError()
```
, then
- ```err instanceof Meteor.Error``` is ```true```
- ```err instanceof FormError``` is ```false```

But if we call ```super``` constructor from ```FormError``` constructor, it works. We call it anyway because it is used to construct ```Meteor.Error``` inside ```FormError```. So, it works fine this way.