Make flags file format errors easier to understand.

Fix json tests.

Author: pborman

file.go

@@ -203,7 +203,7 @@ func (f *Flags) Set(value string, opt getopt.Option) error {
 		// we can re-play after the subset is registered.
 		m, err := f.Decoder(data)
 		if err != nil {
-			return err
+			return fmt.Errorf("%s: %v", value, err)
 		}
 		f.m = mergemap(f.m, m)
 	}

json/json.go

@@ -1,4 +1,4 @@
-// Copyright 2018 Paul Borman
+// Copyright 2019 Paul Borman
 //
 // Licensed under the Apache License, Version 2.0 (the "License");
 // you may not use this file except in compliance with the License.
@@ -31,6 +31,7 @@ package json
 import (
 	"bytes"
 	"encoding/json"
+	"fmt"
 
 	"github.com/pborman/options"
 )
@@ -45,7 +46,7 @@ func Decoder(data []byte) (map[string]interface{}, error) {
 	m := map[string]interface{}{}
 	for decoder.More() {
 		if err := decoder.Decode(&m); err != nil {
-			return nil, err
+			return nil, fmt.Errorf("JSON decoding error: %v", err)
 		}
 	}
 	return m, nil

json/json_test.go

@@ -5,10 +5,10 @@ import (
 	"fmt"
 	"io/ioutil"
 	"os"
-	"pkg/prb/uuid"
 	"reflect"
 	"testing"
 
+	"github.com/google/uuid"
 	getopt "github.com/pborman/getopt/v2"
 	"github.com/pborman/options"
 )