fix(web): surface Forth messages from host-word throws

A host word signals failure by throwing across the JS boundary, and the
browser runtime reported that exception with its Debug form, so an
empty-stack RESIZE surfaced as

    call_func(134) failed: JsValue(Error: Stack underflow ...)

with the engine's JS stack trace glued on. The thrown message IS the
Forth message, so take it verbatim: 'Stack underflow', the same text the
native CLI prints. Exceptions without a message keep the call context --
those are genuine runtime faults, not Forth throws.

CATCH is unaffected: it reads the throw code from its own channel rather
than parsing messages. Verified against a fresh VM in Node (initSync +
WaferRepl): host-word underflow, compiled-guard underflow, THROW,
unknown word and ' RESIZE CATCH . all match the native CLI.

Ships as v0.2.4.

Cargo.toml

@@ -3,7 +3,7 @@ members = ["crates/*"]
 resolver = "2"
 
 [workspace.package]
-version = "0.2.3"
+version = "0.2.4"
 edition = "2024"
 license = "MIT OR Apache-2.0"
 repository = "https://github.com/ok2/wafer"