This is underappreciated! I suppose you are less likely to care about errors if you aren't getting values from a function, but not always. I wonder if there is a proposal to force this case to be handled, like Haskell's -Wunused-do-bind?
There's go vet. But it's inconsistent with the Go warnings-are-errors philosophy. Ignoring the return value of os.Mkdir() is far more likely to be a bug than an unused import is.