Add example for encrypting existing user tokens to the oauth-encryption readme.

Replace the `Meteor._insecureExceptions` flag with
`Meteor._printDecryptionFailure` to more closely match livedata's
`Meteor._printSentDDP` and `Meteor._printReceivedDDP`.  This also has
the advantage of not changing the behavior of the `open` function when
additional debugging is needed in development.