fish: help command return error for non-existent commands (RHBZ#597145).

fish: help command return error for non-existent commands (RHBZ#597145).

With this change, the exit status indicates error for non-existent
commands.

$ guestfish -h foo
foo: command not known, use -h to list all commands
$ echo $?
1
$ guestfish help foo
foo: command not known, use -h to list all commands
$ echo $?
1

For stable-1.2 branch:
 - cherry picked from commit f2b7a8e15c49ebc70c7ea56aefb340362aae5a99
 - rebased for the 1.2 branch