fix missingFileHandler (#440)

- Fix panics when `missingFileHandler` is not defined
- Fix `missingFileHandler: Error` had been writing errors at the Warn level
- Add `Info` and `Debug` as valid values

Ref https://github.com/roboll/helmfile/pull/439#issuecomment-456155521

docs/writing-helmfile.md

@@ -72,6 +72,8 @@ templates:
     chart: stable/{{`{{ .Release.Name }}`}}
     namespace: kube-system
     # This prevents helmfile exiting when it encounters a missing file
+    # Valid values are "Error", "Warn", "Info", "Debug". The default is "Error"
+    # Use "Debug" to make missing files errors invisible at the default log level(--log-level=INFO)
     missingFileHandler: Warn
     values:
     - config/{{`{{ .Release.Name }}`}}/values.yaml