fix go vet nits

main.go

@@ -200,7 +200,7 @@ func (cfg *Config) process() (code []byte, err error) {
 	opt := &imports.Options{Comments: true, TabIndent: true, TabWidth: 8}
 	code, err = imports.Process("", code, opt)
 	if err != nil {
-		panic(fmt.Errorf("BUG: can't gofmt generated code:", err))
+		panic(fmt.Errorf("BUG: can't gofmt generated code: %v", err))
 	}
 	return code, nil
 }
@@ -226,7 +226,7 @@ func loadPackage(cfg *Config) (*types.Package, error) {
 
 func generate(mtyp *marshalerType, cfg *Config) ([]byte, error) {
 	w := new(bytes.Buffer)
-	fmt.Fprintln(w, "// generated by gencodec, do not edit.\n")
+	fmt.Fprint(w, "// generated by gencodec, do not edit.\n\n")
 	fmt.Fprintln(w, "package", mtyp.orig.Obj().Pkg().Name())
 	fmt.Fprintln(w)
 	mtyp.scope.writeImportDecl(w)