EC2 script should exit with non-zero code on UsageError

This is specially import because some ssh errors are raised as UsageError, preventing an automated usage of the script from detecting the failure.

Author: Allan Douglas R. de Oliveira <allan@chaordicsystems.com>

Closes #638 from douglaz/ec2_exit_code_fix and squashes the following commits:

5915e6d [Allan Douglas R. de Oliveira] EC2 script should exit with non-zero code on UsageError
(cherry picked from commit bcb9b7f)

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

ec2/spark_ec2.py

@@ -815,6 +815,7 @@ def main():
     real_main()
   except UsageError, e:
     print >> stderr, "\nError:\n", e
+    sys.exit(1)
 
 
 if __name__ == "__main__":