verify_parse_tree and exceptions

[alessiostalla]

In the KolasuParser.verifyParseTree method (verify_parse_tree in Pylasu), we traverse the parse tree to search for exceptions and add issues if we find them.
I didn't add that as part of #42 because the parse-tree traversing code is missing from Pylasu.
We should aim for feature and behavior parity with Kolasu as much as possible. However, in this case, it looks to me like a duplicate of the parser and lexer error listeners. I think we should have a conversation on whether to keep this in Kolasu 1.7 (and consequently add it to Pylasu) or to remove this check because it's redundant.