[SPARK-11249][LAUNCHER] Throw error if app resource is not provided.

Without this, the code would build an invalid spark-submit command line,
and a more cryptic error would be presented to the user. Also, expose
a constant that allows users to set a dummy resource in cases where
they don't need an actual resource file; for backwards compatibility,
that uses the same "spark-internal" resource that Spark itself uses.

Tested via unit tests, run-example, spark-shell, and running the
thrift server with mixed spark and hive command line arguments.

Author: Marcelo Vanzin <vanzin@cloudera.com>

Closes #12909 from vanzin/SPARK-11249.