Fixes the 'max call stack size exceeded' for reading error message from nested schema

[knacharam-rythmos]

#57

[knacharam-rythmos]

Test cases are passing with mongoose@5.3.1

[knacharam-rythmos]

Hi @matteodelabre,

Build was failed with below error.

WARNING: The following packages cannot be authenticated!
  mongodb-org-server
E: There were unauthenticated packages and -y was used without --allow-unauthenticated
apt-get.diagnostics
apt-get install failed
...
Reading package lists...
W: GPG error: http://repo.mongodb.org/apt/ubuntu trusty/mongodb-org/3.4 Release: The following signatures were invalid: KEYEXPIRED 1515625755
W: The repository 'http://repo.mongodb.org/apt/ubuntu trusty/mongodb-org/3.4 Release' is not signed.
W: http://ppa.launchpad.net/couchdb/stable/ubuntu/dists/trusty/Release.gpg: Signature by key 15866BAFD9BCC4F3C1E0DFC7D69548E1C17EAB57 uses weak digest algorithm (SHA1)
The command "sudo -E apt-get -yq --no-install-suggests --no-install-recommends $(travis_apt_get_options) install mongodb-org-server" failed and exited with 100 during .

Can you rebuild it.