Fix the error message when incorrect number of arguments passed.

Before this change, if there were too little arguments e.g.
`trousseau set abc`, it would print a "Not enough argument supplied ..."
message which is correct but it would print the same message if there
were too many arguments e.g. `trousseau get abc 123`. Also, it should
have been arguments, not argument. This change makes the error message
more accurate.

Please have a look. Thanks :)