follow-up on deprecation of `stopf`

[sorhawell]

It appears that in #468 stopf was a bit hastily replaced with ´stop()` without fixing error msgs.

stopf had a sprintf-like interface to build err msgs, whereas stop as a paste-like interface.

e.g. this does not make sense with stop stop("type of x is not double or integer, it was [%s]", typeof(x))

but should be something like stop("type of x is not double or integer, it was [", typeof(x),"]")

a fast text search on %s reveals about ~30 error messages that need refactoring