allow rsx to be used outside of a proc_macro context

packages/rsx/src/errors.rs

@@ -1,15 +1,11 @@
 macro_rules! missing_trailing_comma {
     ($span:expr) => {
-        proc_macro_error::emit_error!($span, "missing trailing comma")
+        return Err(Error::new($span, "missing trailing comma"));
     };
 }
 
 macro_rules! attr_after_element {
     ($span:expr) => {
-        proc_macro_error::emit_error!(
-            $span,
-            "expected element";
-            help = "move the attribute above all the children and text elements"
-        )
+        return Err(Error::new($span, "expected element\n  = help move the attribute above all the children and text elements"));
     };
 }