encoding/json: address go vet reports

The error message for decoding a unquoted value into a struct field with
the ,string option specified has two arguments when one is needed.
Make the error message take one argument and add a test in order to cover
the case when a unquoted value is specified.

Also add error value as the missing argument for Fatalf call in test.

Fixes the following go vet reports:

decode.go:602: wrong number of args for format in Errorf call: 1 needed but 2 args
decode_test.go:1088: missing argument for Fatalf("%v"): format reads arg 1, have only 0 args

Change-Id: Id036e10c54c4a7c1ee9952f6910858ecc2b84134
Reviewed-on: https://go-review.googlesource.com/2109
Reviewed-by: Mikio Hara <mikioh.mikioh@gmail.com>