[database][js] Reject promise instead of resolving an error object

[simonbengtsson]

Currently ref.once() don't reject properly when a firebase call fails. This pull request fixes this and allows developers to properly handle those cases.

It could probably easiest be demonstrated by disallowing a path in in the database rules and than try a query like this:

firebase.database()
    .ref(`not/allowed/path`)
    .once('value')
    .then(snap => {
      console.log(snap instanceof Error); // true
      console.log(snap.val); // undefined (because it is not a snapshot...)
    })
    .catch(err => {
      // Never called, but is expected to be called
    });

This pull request aline with the behavior of the firebase web sdk.

[chrisbianca]

Great spot!