Report Exception#message instead of Exception#to_s

Because some of libraries override `Exception#message` rather than
`Exception#to_s` and `Exception#message` by default returns `Exception#to_s`
if it's not overridden. So it's better to report `Exception#message`.

Author: kateinoigakukun

packages/npm-packages/ruby-wasm-wasi/src/vm.ts

@@ -602,7 +602,7 @@ class RbExceptionFormatter {
     }
 
     try {
-      message = error.toString();
+      message = error.call("message").toString();
     } catch (e) {
       message = "unknown";
     }