Add ParseException for unassigned const variables

[JackWilliams-FractureSounds]

should be fix in comp_extras.ASTVisitorCheckDeclarations

on init
    declare const name
end on

[mkruselj]

Yeah there actually is a raise ParseException there but it's in the wrong place, it should be outside of the big if clause I think. But also that whole if clause (if 'const' in node.modifiers:) seems quite a bit suspicious, I am not sure I can get any of those parse exceptions to show up by doing literal things those messages are reporting about...