Better error-handling for WASM Python evaluator tests
When our input has syntactical or semantical error(s) in the (Rust)Python evaluator, we get an unhelpful error:
[error] execution failed: unreachable, Code: 0x89\\n
[error] When executing function name: "_start"
We want to replace the enigmatic message we get now with something more informative such as "'result' is undefined'".
(Note: this is not an issue for WASM JS.)
Bug: T356715