[Rd] confusing all.equal output

[Antoine Fabri]

Good points. I don't mind the terminology since target and current are the
names of the arguments. As the function is already designed to stop at the
first failing check we might not need to enumerate or count the mismatches,
instead we could have "`NA` found in `target` but not in `current` at
position <FIRST_MISMATCH>"

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