Improve error message for undefined template key (#1032)

* Add custom error message for undefined key while rendering

* Code refactor

* Fix Lint

* Code Refactor

* Change Undefined key error message

* Update pkg/param/render.go

Co-authored-by: Vivek Singh <vsingh.ggits.2010@gmail.com>

* Update error message

* Use sprintf

* Minor fix

* Fix gofmt issues

* Update pkg/param/render.go

Co-authored-by: Pavan Navarathna <pavan@kasten.io>

Co-authored-by: Vivek Singh <vsingh.ggits.2010@gmail.com>
Co-authored-by: Pavan Navarathna <pavan@kasten.io>

pkg/param/render.go

@@ -16,7 +16,9 @@ package param
 
 import (
 	"bytes"
+	"fmt"
 	"reflect"
+	"strings"
 	"text/template"
 
 	"github.com/Masterminds/sprig"
@@ -25,6 +27,10 @@ import (
 	crv1alpha1 "github.com/kanisterio/kanister/pkg/apis/cr/v1alpha1"
 )
 
+const (
+	undefinedKeyErrorMsg = "map has no entry for key "
+)
+
 // RenderArgs function renders the arguments required for execution
 func RenderArgs(args map[string]interface{}, tp TemplateParams) (map[string]interface{}, error) {
 	ram := make(map[string]interface{}, len(args))
@@ -119,11 +125,24 @@ func renderStringArg(arg string, tp TemplateParams) (string, error) {
 	}
 	buf := bytes.NewBuffer(nil)
 	if err = t.Execute(buf, tp); err != nil {
+		// Check if Error is because of undefined key,
+		// which will lead on execute error on first undefined (missingkey=error).
+		// Get the undefined key name from the error message.
+		if strings.Contains(err.Error(), undefinedKeyErrorMsg) {
+			return "", newUndefinedKeyError(err.Error())
+		}
 		return "", errors.WithStack(err)
 	}
 	return buf.String(), nil
 }
 
+func newUndefinedKeyError(err string) error {
+	pos := strings.LastIndex(err, undefinedKeyErrorMsg)
+	adjustedPos := pos + len(undefinedKeyErrorMsg)
+	key := strings.Trim(err[adjustedPos:], "\"")
+	return errors.WithStack(errors.New(fmt.Sprintf("Failed to render template: \"%s\" not found", key)))
+}
+
 // RenderObjectRefs function renders object refs from TemplateParams
 func RenderObjectRefs(in map[string]crv1alpha1.ObjectReference, tp TemplateParams) (map[string]crv1alpha1.ObjectReference, error) {
 	out := make(map[string]crv1alpha1.ObjectReference, len(in))