Merge pull request #470 from tgravescs/fix_spark_examples_yarn

Only log error on missing jar to allow spark examples to jar.

Right now to run the spark examples on Yarn you have to use the --addJars option and put the jar in hdfs.  To make that nicer  so the user doesn't have to specify the --addJars option change it to simply log an error instead of throwing.
(cherry picked from commit 792d9084e2bc9f778a00a56fa7dcfe4084153aea)

Signed-off-by: Patrick Wendell <pwendell@gmail.com>