The neutron l3 api-def was rehomed in [1], but didn't
include the respective exception classes from neutron's
l3 module.
This patch rehomes those exceptions; they are used by
various sub-projects outside neutron. As the exceptions
are not API facing, they are moved into a new module
under neutron_lib.exceptions.*
[1] https://review.openstack.org/411974/
Change-Id: I2324a3a02789c798248cab41c278a2d9981d24be