Document exception handling for thrown non-Error values

This might be an interesting interop related use-case that is not covered in the docs yet:

let throwPlainValue: unit => unit = %raw(`
function() {
  throw "test"
}
`)

try {
  throwPlainValue()
} catch {
| Js.Exn.Error(obj) =>
  switch Js.Exn.message(obj) {
  | Some(m) => Js.log("Caught a JS exception! Message: " ++ m)
  | None => Js.log2("probably not an error object", obj) // <-- we will hit this  case here
  }
}

In the example above, obj doesn't necessarily mean it's a Error, and ReScript doesn't do any special convertion to make it an Error. So in the None case, the user needs to figure out themselves what kind of value they are dealing with.

This might be problematic in situations where you are e.g. using some library that defines its own weird throwing system (not throwing error, but e.g. plain objects), and need to interact with the value.

Start with the ReScript and JavaScript interop example in the issue, then locate the existing exception-handling documentation. Document that thrown values may be non-Error values and explain the handling shown by the example; done means this interop case is covered clearly for users.