Handle invalid Lisp expression parsing

##### Question LeetCode 736. Parse Lisp Expression – Given a string representing a Lisp-like expression containing let/add/mult operations and variable bindings, evaluate the expression and return the integer result. In addition to the standard problem, the input string may be syntactically invalid; detect invalid cases (e.g., mismatched parentheses, unknown tokens) and return an error indicator. https://leetcode.com/problems/parse-lisp-expression/description/