gh-123440: Improve error message for except as used with not a name

[sobolevn]

The main idea is that this rule is the last in chain. So, any valid cases with NAME expressions are already matched. So, I guess it is safe to use 'except' expression 'as' a=expression later on.

I tried to use a single rule in invalid_except_stmt like:

    | 'except' '*'? expression 'as' a=expression {
        RAISE_SYNTAX_ERROR_KNOWN_LOCATION(
            a, "cannot use except statement with %s", _PyPegen_get_expr_name(a)) }

but since it is used for both except_block and except_star_block - this produced an incorrect result.

• Issue: Improve error message for except a as b.c: case #123440

[lysnikolaou]

LGTM! Thanks @sobolevn!

[pablogsal]

Fantastic! Great work and thanks for the comprehensive test cases 🤘