var-get should throw a runtime error if variable does not exist

[csgui]

For now, var-get on an non-existing variable could lead to the generation of invalid Wasm code.

To remedy this, there is an "easy enough" fix to write:

1. Change the linked function link_get_variable_fn to write an optional Value to the memory;
2. Adapt the traverse function of get-var to unwrap the optional, and return a runtime error if it's none.