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