afr: replace afr_more_important_error with afr_most_important_error

afr_more_important_error() is written to return whether a new errno
should override an existing errno for high-level operations that
could span multiple sub-operations. It specifically prioritizes
ESTALE over EIO over ENOENT, and otherwise defaults to the latest
error passed having priority.

This change preserves current behavior, but rewrites the logic to
return the higher priority error of the existing and new errno. The
purpose of the change is to make the logic a bit more clear and set
the stage for future changes to make the logic flexible based on
context.

BUG: 892730
Change-Id: Id1aa48855dfb0507abc9d1ef22f2259b30472576
Signed-off-by: Brian Foster <bfoster@redhat.com>
Reviewed-on: http://review.gluster.org/4375
Reviewed-by: Jeff Darcy <jdarcy@redhat.com>
Tested-by: Gluster Build System <jenkins@build.gluster.com>