bash doesn't really propagate errors from subshells when using redirection
the way spark-class does; so, instead, this change captures the exit code
of the launcher process in the command array, and checks it before executing
the actual command.

Tested by injecting an error in Main.java (the launcher entry point) and
verifying the shell gets the right exit code from spark-class.

Author: Marcelo Vanzin <vanzin@cloudera.com>

Closes #12910 from vanzin/SPARK-13670.