Set *err to -2 when the list isn't found in tmp_dcursor_init() (#4571)

Set *err to -2 when the list isn't found in tmp_dcursor_init() (#4571)

This change honors the comments describing how err is handled, and
also means, given tmpl_find_vp()'s use of tmpl_dcursor_init(),
means callers of tmpl_find_vp() can check its return value
rather than having to look at the written vp (e.g. sometimes_return();
see CID #1503995).