SPARK-3014. Log a more informative messages in a couple failure scenario...

...s
This language is quite specific to the internal implementation and might be a bit confusing for users. For instance, what does it mean to "retrieve" a SparkContext? It might be better to make it more consistent with the language in the two other log statements and say something like:

```
Spark context did not initialize properly after waiting for XXX.
Please check earlier log output for errors.
Failing the application.
```

Also - I don't think this will print a unit for the wait time. I believe the unit is `ms` so it might be good to add that.