Go 1.5 and earlier said "day out of range".
As part of working on this code it morphed into "day of month out of range".
To avoid churn in the output restore the old text.

This fixes some tests reported privately.

Change-Id: If179676cd49f9a471a9441fec2f5220c85eb0799
Reviewed-on: https://go-review.googlesource.com/18386Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>