spark-core - [SPARK-4787] - Stop sparkcontext properly if a DAGScheduler init error occurs

[SPARK-4787] Stop SparkContext properly if an exception occurs during DAGscheduler initialization.
Style nit: you can use string interpolation instead of String.format, which will allow the `new SparkException` to fit on the same line as `throw`:

```scala
throw new SparkException("DAGScheduler cannot be initialized due to ${e.getMessage}")
```

However, I'd prefer to call the two-argument constructor which takes the cause as second argument, since this will lead to more informative stacktraces:

```scala
throw new SparkException("Error while constructing DAGScheduler", e)
```