Check return value of lsu_com_help().

Check return value of lsu_com_help().

This function fails if an invalid command name is passed as the argument, yet
all callers ignore the error. Modify the callers to print the strerror text as
appropriate and no longer do that in lsu_lopsub_error(). Rename this function
and introduce the error type argument to print more meaningful error messages.

One visible consequence is that

	para_client help does-not-exist

used to succeed while it now exits with status 1.