Detect when wasmtime is not installed in `Tools/wasm/wasi.py`

[brettcannon]

Feature or enhancement

Proposal:

Right now, Tools/wasm/wasi.py calls shutil.which() at the global level of the file. That means you can end up w/ None in your HOSTRUNNER environment variable.

Has this already been discussed elsewhere?

This is a minor feature, which does not need previous discussion elsewhere

Links to previous discussion of this feature:

No response

Linked PRs

• GH-121521: Detect when wasmtime is not installed in Tools/wasm/wasi.py #121522
• [3.13] GH-121521: Detect when wasmtime is not installed in Tools/wasm/wasi.py (GH-121522) #121558