When there's more than one argument missing, the message miscounts the number of
required arguments. For instance, add this to testdata/print.go:
  Printf("%s %d %v %q", "str", 4) // ERROR "missing argument for Printf verb %v: need 4, have 2"

vet flags an issue with the %v, but says "need 3, have 2" because it is
progressively checking the Printf call instead of parsing the entire format first, and
checking the correspondence second.