Fix several issues found by 'go vet'

I ran `go vet ./...` and have fixed all of the following issues:

```
[zsh|matt@nerr-2]:~/git/go/src/github.com/bradfitz/http2 0 (master) ± go vet ./...
errors_test.go:24: unrecognized printf verb 'i'
frame_test.go:72: arg df.Data for printf verb %q of wrong type: func() []byte
frame_test.go:347: missing argument for Errorf("%v"): format reads arg 4, have only 3 args
server_test.go:508: missing argument for Errorf("%2f"): format reads arg 2, have only 1 args
server_test.go:1899: missing argument for Errorf("%d"): format reads arg 1, have only 0 args
exit status 1
hpack/encode_test.go:84: possible formatting directive in Error call
hpack/hpack_test.go:130: arg got for printf verb %q of wrong type: hpack.HeaderField
hpack/hpack_test.go:135: arg got for printf verb %q of wrong type: hpack.HeaderField
hpack/hpack_test.go:138: arg got for printf verb %q of wrong type: hpack.HeaderField
hpack/hpack_test.go:141: arg got for printf verb %q of wrong type: hpack.HeaderField
hpack/hpack_test.go:199: arg got for printf verb %q of wrong type: hpack.HeaderField
exit status 1
```

In addition, I have already signed the Google Individual CLA under the same email address: mdlayher@gmail.com.  Thanks again for your time!