strutils: move an error message to where it is used, and improve it

strutils: move an error message to where it is used, and improve it

Calling gettext() is somewhat costly: it has to find the given message
among the more than five thousand messages in util-linux's repertoire.
So, call gettext() only when the message actually gets printed.

Besides, allowing to customize the error message for parse_switch() was
a nice gesture, but it's unneeded: a fixed error message is good enough.

Also, "argument error" was rather vague, as it doesn't say _what_ the
error is.  Better say "unsupported argument".

Signed-off-by: Benno Schulenberg <bensberg@telfort.nl>