docs: Add error to smart subscription docs (#632)

Author: darrylhein

docs/api/smart-subscription.md

@@ -9,6 +9,7 @@ Each subscription declared in the `apollo.$subscribe` option in a component resu
 - `throttle`: throttle variables updates (in ms).
 - `debounce`: debounce variables updates (in ms).
 - `result(data, key)` is a hook called when a result is received
+- `error(error)` is a hook called when there are errors. `error` is an Apollo error object with either a `graphQLErrors` property or a `networkError` property.
 
 ## Properties