expr: Refine handling of error parameter to expr_annotate().

expr: Refine handling of error parameter to expr_annotate().

In most cases expr_annotate() set '*errorp' to NULL if it was successful,
but there was one case where it did not.  This corrects that and refines
the comment to better explain the intended behavior.

This didn't affect any existing users because all of them passed in a
pointer that was already NULL.

Signed-off-by: Ben Pfaff <blp@ovn.org>
Acked-by: Ryan Moats <rmoats@us.ibm.com>