If will see an error if x is defined later.
There's no error message if err will be replace by assert 0.
But if I will use this block of code multiple times (with different variables instead of x) I can see many error messages.
But I want to stop on the 1st error message.
How can I do this?

Both assert 0 and err will cause assembly to stop. But assert is a "soft" stop, and err is a "hard" stop.

Assert will allow more passes to resolve variables before erroring.
Err will immediately halt.

Yeah, I understand.
I've defined a const to show only first error with assert

But now I have another problem:

Here I see message "Error!".
qqq generates an error and a becomes undefined. This creates a sense of error in the wrong place.
Is there's a way to avoid "display"ing the custom message in this case.

Move "tst a" further down after the qqq line.

Or change the text to say "x is not defined yet". It isn't actually an error to have undefined variables, so displaying "error" is not strictly correct.