tools: display error context when it exists (#749)

For example when failing to decode toml, display the context around the
error and the location of the problem.

internal/cli/cli.go

@@ -2,11 +2,14 @@ package cli
 
 import (
 	"bytes"
+	"errors"
 	"flag"
 	"fmt"
 	"io"
 	"io/ioutil"
 	"os"
+
+	"github.com/pelletier/go-toml/v2"
 )
 
 type ConvertFn func(r io.Reader, w io.Writer) error
@@ -28,7 +31,16 @@ func (p *Program) Execute() {
 func (p *Program) main(files []string, input io.Reader, output, error io.Writer) int {
 	err := p.run(files, input, output)
 	if err != nil {
-		fmt.Fprintln(error, err.Error())
+
+		var derr *toml.DecodeError
+		if errors.As(err, &derr) {
+			fmt.Fprintln(error, derr.String())
+			row, col := derr.Position()
+			fmt.Fprintln(error, "error occurred at row", row, "column", col)
+		} else {
+			fmt.Fprintln(error, err.Error())
+		}
+
 		return -1
 	}
 	return 0

internal/cli/cli_test.go

@@ -10,6 +10,7 @@ import (
 	"strings"
 	"testing"
 
+	"github.com/pelletier/go-toml/v2"
 	"github.com/stretchr/testify/assert"
 	"github.com/stretchr/testify/require"
 )
@@ -47,6 +48,21 @@ func TestProcessMainStdinErr(t *testing.T) {
 	assert.NotEmpty(t, stderr.String())
 }
 
+func TestProcessMainStdinDecodeErr(t *testing.T) {
+	stdout := new(bytes.Buffer)
+	stderr := new(bytes.Buffer)
+	input := strings.NewReader("this is the input")
+
+	exit := processMain([]string{}, input, stdout, stderr, func(r io.Reader, w io.Writer) error {
+		var v interface{}
+		return toml.Unmarshal([]byte(`qwe = 001`), &v)
+	})
+
+	assert.Equal(t, -1, exit)
+	assert.Empty(t, stdout.String())
+	assert.Contains(t, stderr.String(), "error occurred at")
+}
+
 func TestProcessMainFileExists(t *testing.T) {
 	tmpfile, err := ioutil.TempFile("", "example")
 	require.NoError(t, err)