[SPARK-23240][PYTHON] Better error message when extraneous data in pyspark.daemon's stdout
## What changes were proposed in this pull request? Print more helpful message when daemon module's stdout is empty or contains a bad port number. ## How was this patch tested? Manually recreated the environmental issues that caused the mysterious exceptions at one site. Tested that the expected messages are logged. Also, ran all scala unit tests. Please review http://spark.apache.org/contributing.html before opening a pull request. Author: Bruce Robbins <bersprockets@gmail.com> Closes #20424 from bersprockets/SPARK-23240_prop2.
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