Fix incorrect parser behavior.

[DanilaFe]

Currently, the Shunting Yard based parser works well for all valid expressions (except for implicit grouping). However, it also works for some invalid expressions. For instance, 3+!4 parses to 3!+4, when it shouldn't parse at all. This error isn't caught during parse time because the re-ordered tokens are still valid, and should probably be caught during shunting yard itself.