feat: Better exec error reporting (#1159)

Enhances Helmfile to print more helpful message on error while calling `exec` template function.

Helmfile has been printing error messages like the below:

```
in ./helmfile.yaml: error during helmfile.yaml.part.0 parsing: template: stringTemplate:5:8: executing "stringTemplate" at <exec "./exectest.sha" (list)>: error calling exec: exit status 1
```

Adding captured stdout and stderr, with some indentation to make it readable, it now produces the following message on missing executable:

```
$ make build && ./helmfile build
go build
in ./helmfile.yaml: error during helmfile.yaml.part.0 parsing: template: stringTemplate:5:8: executing "stringTemplate" at <exec "./exectest.sha" (list)>: error calling exec: fork/exec ./exectest.sha: no such file or directory

COMMAND:
  ./exectest.sha

ERROR:
  fork/exec ./exectest.sha: no such file or directory
```

On non-zero exit status without output:

```
$ make build && ./helmfile build
go build
in ./helmfile.yaml: error during helmfile.yaml.part.0 parsing: template: stringTemplate:5:8: executing "stringTemplate" at <exec "./exectest.sh" (list)>: error calling exec: exit status 1

COMMAND:
  ./exectest.sh

ERROR:
  exit status 1
```

On non-zero exit status with output:

```
$ make build && ./helmfile build
go build
in ./helmfile.yaml: error during helmfile.yaml.part.0 parsing: template: stringTemplate:5:8: executing "stringTemplate" at <exec "./exectest.sh" (list)>: error calling exec: exit status 2

COMMAND:
  ./exectest.sh

ERROR:
  exit status 2

COMBINED OUTPUT:
  out1
  err1
```

Resolves #1158

pkg/tmpl/context_funcs.go

@@ -92,9 +92,27 @@ func (c *Context) Exec(command string, args []interface{}, inputs ...string) (st
 	var bytes []byte
 
 	g.Go(func() error {
-		bs, err := cmd.Output()
+		// We use CombinedOutput to produce helpful error messages
+		// See https://github.com/roboll/helmfile/issues/1158
+		bs, err := cmd.CombinedOutput()
 		if err != nil {
-			return fmt.Errorf("exec cmd=%s args=[%s] failed: %v", command, strings.Join(strArgs, ", "), err)
+			args := strings.Join(strArgs, ", ")
+			shownCmd := []string{command}
+			if len(args) > 0 {
+				shownCmd = append(shownCmd, args)
+			}
+
+			var out string
+
+			out += fmt.Sprintf("\n\nCOMMAND:\n%s", Indent(strings.Join(shownCmd, " "), "  "))
+
+			out += fmt.Sprintf("\n\nERROR:\n%s", Indent(err.Error(), "  "))
+
+			if len(bs) > 0 {
+				out += fmt.Sprintf("\n\nCOMBINED OUTPUT:\n%s", Indent(string(bs), "  "))
+			}
+
+			return fmt.Errorf("%v%s", err, out)
 		}
 
 		bytes = bs
@@ -109,6 +127,29 @@ func (c *Context) Exec(command string, args []interface{}, inputs ...string) (st
 	return string(bytes), nil
 }
 
+// indents a block of text with an indent string
+func Indent(text, indent string) string {
+	var b strings.Builder
+
+	b.Grow(len(text) * 2)
+
+	lines := strings.Split(text, "\n")
+
+	last := len(lines) - 1
+
+	for i, j := range lines {
+		if i > 0 && i < last && j != "" {
+			b.WriteString("\n")
+		}
+
+		if j != "" {
+			b.WriteString(indent + j)
+		}
+	}
+
+	return b.String()
+}
+
 func (c *Context) ReadFile(filename string) (string, error) {
 	var path string
 	if filepath.IsAbs(filename) {

pkg/tmpl/context_tmpl.go

@@ -6,7 +6,7 @@ import (
 	"text/template"
 )
 
-func (c *Context) stringTemplate() *template.Template {
+func (c *Context) newTemplate() *template.Template {
 	funcMap := sprig.TxtFuncMap()
 	for name, f := range c.createFuncMap() {
 		funcMap[name] = f
@@ -21,7 +21,7 @@ func (c *Context) stringTemplate() *template.Template {
 }
 
 func (c *Context) RenderTemplateToBuffer(s string, data ...interface{}) (*bytes.Buffer, error) {
-	var t, parseErr = c.stringTemplate().Parse(s)
+	var t, parseErr = c.newTemplate().Parse(s)
 	if parseErr != nil {
 		return nil, parseErr
 	}