Throw an exception for a missing <file_name> param in ansible-vault

Without this fix you have to enter your vault password before you realize
that you forgot to pass in the filename.  This commit checks that an
filename argument was at least passed on the command line.

bin/ansible-vault

@@ -112,7 +112,6 @@ def _read_password(filename):
     return data
 
 def execute_create(args, options, parser):
-
     if len(args) > 1:
         raise errors.AnsibleError("'create' does not accept more than one filename")        
 
@@ -204,6 +203,11 @@ def main():
     parser = build_option_parser(action)
     (options, args) = parser.parse_args()
 
+    if not len(args):
+        raise errors.AnsibleError(
+            "The '%s' command requires a filename as the first argument" % action
+        ) 
+
     # execute the desired action
     try:
         fn = globals()["execute_%s" % action]