command: Fix confusing syntax error message

If the user executes a command with an invalid subcommand, the error
message is extremely unhelpful:

> flash write test.elf
flash write test.elf: command requires more arguments

This is because any command line that starts with a valid command group is
classified as a group, triggering ocd_bouncer to print the confusing
message.

Fix by requiring that to be a command group, the command line must not
contain any unknown tokens after the last valid (sub-)command group. That
is OK because command groups don't have handlers defined and thus can't
take any parameters.

Also fix the error message for "unknown" type to be similar to the error
message that is printed (by Jim) for non-existent primary commands.

Change-Id: I26950349f0909fd3961c4f9ab9b198c221cea9fc
Signed-off-by: Andreas Fritiofson <andreas.fritiofson@gmail.com>
Reviewed-on: http://openocd.zylin.com/2285
Tested-by: jenkins
Reviewed-by: Spencer Oliver <spen@spen-soft.co.uk>