webidl: Turn the [Throws] extended attributes into Result<T, JsValue>

This sets the `catch` flag on the emitted AST when an operation/attribute has
the `[Throws]` extended attribute on it.

Additionally, constructors aren't annotated with `[Throws]` but can still throw
exceptions, so we must conservatively assume *every* constructor can throw an
error.

crates/backend/src/util.rs

@@ -35,6 +35,23 @@ pub fn raw_ident(name: &str) -> Ident {
 /// Create a path type from the given segments. For example an iterator yielding
 /// the idents `[foo, bar, baz]` will result in the path type `foo::bar::baz`.
 pub fn simple_path_ty<I>(segments: I) -> syn::Type
+where
+    I: IntoIterator<Item = Ident>,
+{
+    path_ty(false, segments)
+}
+
+/// Create a global path type from the given segments. For example an iterator
+/// yielding the idents `[foo, bar, baz]` will result in the path type
+/// `::foo::bar::baz`.
+pub fn leading_colon_path_ty<I>(segments: I) -> syn::Type
+where
+    I: IntoIterator<Item = Ident>,
+{
+    path_ty(true, segments)
+}
+
+fn path_ty<I>(leading_colon: bool, segments: I) -> syn::Type
 where
     I: IntoIterator<Item = Ident>,
 {
@@ -49,7 +66,11 @@ where
     syn::TypePath {
         qself: None,
         path: syn::Path {
-            leading_colon: None,
+            leading_colon: if leading_colon {
+                Some(Default::default())
+            } else {
+                None
+            },
             segments: syn::punctuated::Punctuated::from_iter(segments),
         },
     }.into()