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