Diagnose invalid command arguments.

Diagnose invalid command arguments.

This started because I objected to parted failing with no diagnostic
when given an invalid file system type:

    $ /sbin/parted -s $dev mklabel loop mkpartfs hfsplus 0 1.4
    WARNING: You are not superuser.  Watch out for permissions.
    [Exit 1]

With the changes below, it does this:

    $ ./parted -s $dev mklabel loop mkpartfs hfsplus 0 1.4
    ./parted: invalid token: hfsplus
    [Exit 1]

The following may look like a simple change, but looks are deceptive...
For example, if you try to diagnose via ped_exception_throw instead
of "error", you'll find that the mere fact of diagnosing the problem
introduces new ones because of how the exception handler manipulates the
global command line buffer containing the token we're complaining about.

But this isn't library code, so using error() is fine.

	Diagnose invalid command arguments.
	When a command argument doesn't match the expected candidate values,
	parted would silently exit (in script mode) or simply act as if that
	value and any following ones had not been specified (in interactive
	mode).  With this change, it complains about the "invalid token",
	and in script mode (where there hasn't been a prompt to give context)
	sometimes tells what type of token it was expecting.
	* parted/ui.c: Include "error.h".
	(command_line_get_word): If the user's "token" wasn't a good enough
	match, give a diagnostic.  In script mode, return NULL so that the
	callers can give additional information.
	* tests/t2000-mkfs.sh: New test for the above.
	* tests/t0000-basic.sh: Expect the new diagnostic when "msdos" is
	treated as an unrecognized first token after "mklabel".  This happens
	when trying to label a disk that already has a label.
	* tests/t1100-busy-label.sh: Likewise.