report line:ch in live-preview errors for usercss

edit/codemirror-editing-hooks.js

@@ -623,6 +623,10 @@ onDOMscriptReady('/codemirror.js').then(() => {
     }).then(() => {
       errors.classList.add('hidden');
     }).catch(err => {
+      if (err && editor && !Number.isNaN(err.index)) {
+        const pos = editors[0].posFromIndex(err.index);
+        err = `${pos.line}:${pos.ch} ${err}`;
+      }
       errors.classList.remove('hidden');
       errors.onclick = () => messageBox.alert(String(err), 'pre');
     });

js/usercss.js

@@ -450,8 +450,8 @@ var usercss = (() => {
     try {
       doParse();
     } catch (e) {
-      // grab additional info
-      e.index = metaIndex;
+      // the source code string offset
+      e.index = metaIndex + state.re.lastIndex;
       throw e;
     }