mke2fs: improve the error message when a non-existent file is specified

mke2fs: improve the error message when a non-existent file is specified

If the user does not specify the file system size, and the file does
not exist, give an error message like this:

   The file /tmp/foo.img does not exist and no size was specified.

instead of this:

    Creating regular file /tmp/foo.img
    mke2fs: Device size reported to be zero.  Invalid partition specified, or
	    partition table wasn't reread after running fdisk, due to
	    a modified partition being busy and in use.  You may need to reboot
	    to re-read your partition table.

Signed-off-by: Theodore Ts'o <tytso@mit.edu>