[SPARK-3140] Clarify confusing PySpark exception message

We read the py4j port from the stdout of the `bin/spark-submit` subprocess. If there is interference in stdout (e.g. a random echo in `spark-submit`), we throw an exception with a warning message. We do not, however, distinguish between this case from the case where no stdout is produced at all.

I wasted a non-trivial amount of time being baffled by this exception in search of places where I print random whitespace (in vain, of course). A clearer exception message that distinguishes between these cases will prevent future headaches that I have gone through.
Should this be more descriptive? Like "Expected GatewayServer to output a port; found: <actual>" (otherwise I wonder if printing gateway_port + stdout will seem cryptic to people)