Add function to force an nvlist into the error state

Add an nvlist_set_error() function that can be used to force an
nvlist into the error state. This is useful both for writing
tests and for writing APIs that use nvlists internally.

Differential Revision: https://reviews.freebsd.org/D1878
Reviewed by: pjd, jfv
MFC After: 1 month
Sponsored by: Sandvine Inc.