Bugfix for GSA-552

-- https://jira.broadinstitute.org/browse/GSA-552
-- User reports a null exception while using VariantsToVCF:
   http://gatkforums.broadinstitute.org/discussion/1461/nullpointerexception-converting-vcf3-to-vcf-using-variantstovcf
   The problem is that he left out an input VCF file for the --variant argument and the command-line argument parsing code didn't catch this, so we NPE out later on.

public/java/src/org/broadinstitute/sting/commandline/ArgumentTypeDescriptor.java

@@ -406,7 +406,7 @@ public abstract class ArgumentTypeDescriptor {
             else
                 throw new UserException.CommandLineException(
                         String.format("Failed to parse value %s for argument %s. Message: %s",
-                                value.asString(), fieldName, e.getMessage()));
+                                value, fieldName, e.getMessage()));
         }
     }
 }