Ad additional error checking to n_cpu
Ad additional error checking to n_cpu

git-svn-id: ea4449f358ea2d13621e3cf02b49d1c6a64ed7f9@2659 fa922fa7-2fde-0310-acd8-f43f465a7996

joaander/hoomd-blue,joaander/hoomd-blue,joaander/hoomd-blue,joaander/hoomd-blue,joaander/hoomd-blue,joaander/hoomd-blue