Improve error message in Spark jobs if MongoDB is down

## Background
Currently, if MongoDB is down and Menas is up, Standardization and Conformance Jobs report an ambiguous error:

```
Exception in thread "main" za.co.absa.enceladus.dao.DaoException:
Server returned HTTP response code: 500 for Menas URL:
```

## Feature
Improve the error message to make it easier to investigate such issues.

## Proposed Solution [Optional]
One of the ideas proposed by @GeorgiChochov is to change the status code to `503 - Service Unavailable` for MongoDB connection issues.

Menas can provide additional information on what is unavailable in an error message. Standardization and Conformance could add the message in the exception.