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_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
 	}