[SPARK-2242] HOTFIX: Do not mask pyspark stderr from output

This reverts a change introduced in 3870248740d83b0292ccca88a494ce19783847f0 that masked stderr from surfacing to the `bin/pyspark` shell output. By itself this is not a bug. However, if your `spark.master` is not specified correctly, for example, your spark jobs just hang without any output instead of indicating that it cannot connect to the master.

That commit was not merged in branch-1.0, so this fix is for master only.