Add line number to JSON syntax error.

Thanks to markpeek/packer@5bf33a0

containers.go

@@ -1,6 +1,7 @@
 package main
 
 import (
+	"bytes"
 	"encoding/json"
 	"fmt"
 	"io/ioutil"
@@ -26,10 +27,32 @@ func readCranefile(filename string) Containers {
 	return unmarshal(file)
 }
 
+// Thanks to https://github.com/markpeek/packer/commit/5bf33a0e91b2318a40c42e9bf855dcc8dd4cdec5
+func displaySyntaxError(js []byte, syntaxError error) (err error) {
+	syntax, ok := syntaxError.(*json.SyntaxError)
+	if !ok {
+		err = syntaxError
+		return
+	}
+	newline := []byte{'\x0a'}
+	space := []byte{' '}
+
+	start, end := bytes.LastIndex(js[:syntax.Offset], newline)+1, len(js)
+	if idx := bytes.Index(js[start:], newline); idx >= 0 {
+		end = start + idx
+	}
+
+	line, pos := bytes.Count(js[:start], newline)+1, int(syntax.Offset)-start-1
+
+	err = fmt.Errorf("\nError in line %d: %s \n%s\n%s^", line, syntaxError, js[start:end], bytes.Repeat(space, pos))
+	return
+}
+
 func unmarshal(data []byte) Containers {
 	var containers Containers
 	err := json.Unmarshal(data, &containers)
 	if err != nil {
+		err = displaySyntaxError(data, err)
 		panic(err)
 	}
 	return containers