encoding/jsonfile: stutter less (JSON is already mentioned in inner…

… error)

encoding/jsonfile/jsonfile.go

@@ -54,7 +54,9 @@ func unmarshalInternal(source io.Reader, data interface{}, disallowUnknownFields
 	if err := jsonDecoder.Decode(data); err != nil {
 		// sadly, line numbers are only possible with hacks (requiring buffering):
 		//   https://github.com/hashicorp/packer/blob/master/common/json/unmarshal.go
-		return fmt.Errorf("JSON parsing failed: %s", err.Error())
+		//
+		// no need mention JSON as context in error message, because `err` already mentions that.
+		return fmt.Errorf("decode failed: %s", err.Error())
 	}
 
 	return nil

encoding/jsonfile/jsonfile_test.go

@@ -0,0 +1,12 @@
+package jsonfile
+
+import (
+	"testing"
+
+	"github.com/function61/gokit/testing/assert"
+)
+
+func TestRead(t *testing.T) {
+	assert.Equal(t, ReadDisallowUnknownFields("notfound.json", &struct{}{}).Error(), "open notfound.json: no such file or directory")
+	assert.Equal(t, ReadDisallowUnknownFields("testdata/example.json", &struct{}{}).Error(), `testdata/example.json: decode failed: json: unknown field "foo"`)
+}

encoding/jsonfile/testdata/example.json

@@ -0,0 +1,3 @@
+{
+	"foo": "bar"
+}