[SPARK-4472][Shell] Print "Spark context available as sc." only when SparkContext is created...

... successfully

It's weird that printing "Spark context available as sc" when creating SparkContext unsuccessfully.


> inside your change, we should use echo instead of println.

But it's a scala statement. Note `echo` is a method of `SparkILoop`. Do you mean calling `org.apache.spark.repl.Main.interp.echo("...")`? Then I prefer `println`