Missing validation for unresolved references in conditionals

[mhasel]

Describe the bug
When a reference in an IF-statement condition is not resolvable, we do not currently catch that during validation. Instead, the user receives a rather cryptic cannot generate call statement for ReferenceExpr { kind: Member(Identifier { name: "UINT_EQUAL" }), base: None } at: ....

To Reproduce
Steps to reproduce the behavior:
compile the following:

TYPE ErrorCode :
STRUCT
    Lib     : UINT;     (**< Error library code *)
    Reason  : UINT;     (**< Error reason code *)
    Func    : UDINT;    (**< Error function code *)
END_STRUCT
END_TYPE

PROGRAM mainProg
VAR
    Error: ErrorCode;
END_VAR

IF Error.Reason <> ERROR_OK THEN // unresolved reference in conditional

END_IF
END_PROGRAM

Expected behavior
A clear, concise and helpful error message about the unresolved reference.

[mhasel]

Here's a similar, even more convoluted example:

TYPE myEnum : (
    GO,
    STOP
);
END_TYPE

FUNCTION main : DINT
VAR
    count: DINT;
END_VAR
    WHILE (state = GO OR GO = GO) DO // must be a nested binary expression to reproduce, hence the `GO = GO`
        count := count +1;
    END_WHILE;
END_FUNCTION

Trying to compile this results in the following error:

cannot generate call statement for ReferenceExpr { kind: Member(Identifier { name: "VOID_EQUAL" }), base: None } at: ...

In the above example, state is an unresolved reference. It seems like our validations for binary expressions need some work.