More robust against missing arguments.
More robust against missing arguments.

After watching -jvm-debug hang and scratching my head,
added defense against missing mandatory args.

mdedetrich/sbt,mdedetrich/sbt,sbt/sbt,sbt/sbt,xuwei-k/xsbt,sbt/sbt-launcher-package,sbt/sbt,sbt/sbt,yeghishe/sbt-extras,mdedetrich/sbt,mdedetrich/sbt,sbt/sbt,xuwei-k/xsbt,BanzaiMan/sbt-extras,mdedetrich/sbt,mdedetrich/sbt,xuwei-k/xsbt,xuwei-k/xsbt,mdedetrich/sbt,schleichardt/sbt-extras,xuwei-k/xsbt,sbt/sbt,sbt/sbt,sbt/sbt-launcher-package