update-ref --stdin: harmonize error messages

update-ref --stdin: harmonize error messages

Make (most of) the error messages for invalid input have the same
format [1]:

    $COMMAND [SP $REFNAME]: $MESSAGE

Update the tests accordingly.

[1] A few error messages are left with their old form, because
    $COMMAND and $REFNAME aren't passed all the way down the call
    stack.  Maybe those sites should be changed some day, too.

Signed-off-by: Michael Haggerty <mhagger@alum.mit.edu>
Signed-off-by: Junio C Hamano <gitster@pobox.com>