improve error handling for value rendering (#235)

Fixes #233

Output on values render error:

```
err: failed to render [/Users/sstarcher/xxx/xxx/values.yaml], because of template: stringTemplate:10:18: executing "stringTemplate" at <requiredEnv "HELM_AC...>: error calling requiredEnv: required env var `HELM_ACCOUNT` is not set
```

Also removes panic and sets the output as `apps.Run()` can and will return errors.  Panic makes no sense.

Changelog:

* improve error handling for value rendering

* only output if error exists

* add exit status

main.go

@@ -386,7 +386,8 @@ func main() {
 
 	err := app.Run(os.Args)
 	if err != nil {
-		log.Panicf("[bug] this code path shouldn't be arrived: helmfile is expected to exit from within the `cleanup` func in main.go: %v", err)
+		logger.Errorf("%v", err)
+		os.Exit(3)
 	}
 }
 

state/state.go

@@ -703,9 +703,11 @@ func (state *HelmState) namespaceAndValuesFlags(helm helmexec.Interface, basePat
 			if _, err := os.Stat(path); os.IsNotExist(err) {
 				return nil, err
 			}
+
 			yamlBuf, err := RenderTemplateFileToBuffer(path)
 			if err != nil {
-				return nil, err
+
+				return nil, fmt.Errorf("failed to render [%s], because of %v", path, err)
 			}
 			valfile, err := ioutil.TempFile("", "values")
 			if err != nil {