Move error information functions to instance.c

This reduces the size of the code

Change-Id: I9297fe54806d96554a6ce9ceaf34e1744d0b1d87
Reviewed-on: http://review.couchbase.org/39927
Tested-by: Mark Nunberg <mnunberg@haskalah.org>
Reviewed-by: Brett Lawson <brett19@gmail.com>