setup: update error message to be more meaningful

setup: update error message to be more meaningful

The error message shown when a flag is found when expecting a
filename wasn't clear as it didn't communicate what was wrong
using the 'suitable' words in *all* cases.

        $ git ls-files
        README.md
        test-file

Correct case,

        $ git rev-parse README.md --flags
        README.md
        --flags
        fatal: bad flag '--flags' used after filename

Incorrect case,

        $ git grep "some random regex" -n
        fatal: bad flag '-n' used after filename

The above case is incorrect as "some random regex" isn't a filename
in this case.

Change the error message to be general and communicative. This results
in the following output,

        $ git rev-parse README.md --flags
        README.md
        --flags
        fatal: option '--flags' must come before non-option arguments

        $ git grep "some random regex" -n
        fatal: option '-n' must come before non-option arguments

Signed-off-by: Kaartic Sivaraam <kaarticsivaraam91196@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>