Relax alias test for failing command

The alias test "exprf x 1234" expands to "expr -f x 1234" and is
expected to fail: it ends up trying to evaluate the invalid expression

    void
    $__lldb_expr(void *$__lldb_arg)
    {
        -f x 1234;
    }

On FreeBSD LLDB ends up finding a static function f() in a math library,
and thus the error produced does not include "use of undeclared
identifier 'f'".

We will report failure to parse the expression in any case, so require
only that error message.