Output fancier error message on parse error

3x5.js

@@ -25,7 +25,11 @@
  */
 function renderCard(state, code) {
   const script = Notcl.parse(code);
-  state.output = JSON.stringify(script, null, 2);
+  if (script[0]) {
+    state.output = JSON.stringify(script[1], null, 2);
+  } else {
+    state.output = script[1];
+  }
   return state.output;
 }
 

notcl.js

@@ -64,8 +64,8 @@ var Notcl = (() => {
      * Parse out a Notcl script into an easier-to-interpret representation.
      * No script is actually executed yet.
      *
-     * @param {string} code
-     * @returns Script
+     * @param {string} code to parse
+     * @returns {[true, Notcl.Script] | [false, string]} parsed list of commands, or error message on failure
      */
     parse(code) {
       /* Preprocess */
@@ -85,9 +85,9 @@ var Notcl = (() => {
         );
         return [
           false,
-          `Error at position ${commands[1]}
-${before}${after}
-${"-".repeat(before.length)}^`,
+          `<pre>Error at position ${commands[1]}
+${escapeHtml(before + "" + after)}
+${"-".repeat(before.length)}^</pre>`,
         ];
       }
     },