feat templater: Fail if values are missing

Golang's template package now has an option for failing if template variables
are missing: https://golang.org/pkg/text/template/#Template.Option

This updates the templater code to make use of that option and return the
errors encountered during templating.

This fixes #1

templater/templater_test.go

@@ -3,6 +3,7 @@ package templater
 import (
 	"github.com/tazjin/kontemplate/context"
 	"reflect"
+	"strings"
 	"testing"
 )
 
@@ -136,3 +137,19 @@ func TestApplyLimitsExcludeIncludePrecedence(t *testing.T) {
 		t.Fail()
 	}
 }
+
+func TestFailOnMissingKeys(t *testing.T) {
+	ctx := context.Context{}
+	resourceSet := context.ResourceSet{}
+
+	_, err := templateFile(&ctx, &resourceSet, "testdata/test-template.txt")
+
+	if err == nil {
+		t.Errorf("Template with missing keys should have failed.\n")
+		t.Fail()
+	}
+
+	if !strings.Contains(err.Error(), "map has no entry for key \"testName\"") {
+		t.Errorf("Templating failed with unexpected error: %v\n", err)
+	}
+}