Introduce a TreeSitterError atom kind

Starting work on Wilfred#267

src/display/style.rs

@@ -299,6 +299,9 @@ pub fn color_positions(
                             AtomKind::Keyword | AtomKind::Type => {
                                 style = style.bold();
                             }
+                            AtomKind::TreeSitterError => {
+                                style = style.purple()
+                            }
                             AtomKind::Normal => {}
                         }
                     }

src/parse/syntax.rs

@@ -583,6 +583,7 @@ pub enum AtomKind {
     Type,
     Comment,
     Keyword,
+    TreeSitterError,
 }
 
 /// Unlike atoms, tokens can be delimiters like `{`.

src/parse/tree_sitter_parser.rs

@@ -1238,13 +1238,18 @@ fn atom_from_cursor<'a>(
         content = content.trim();
     }
 
-    // 'extra' nodes in tree-sitter are comments. Most parsers use
-    // 'comment' as their comment node name, but if they don't we can
-    // still detect comments by looking at their syntax highlighting.
-    let highlight = if node.is_extra()
+    node.is_error();
+
+    let highlight = if node.is_error() {
+        AtomKind::TreeSitterError
+    } else if node.is_extra()
         || node.kind() == "comment"
         || highlights.comment_ids.contains(&node.id())
     {
+        // 'extra' nodes in tree-sitter are comments. Most parsers use
+        // 'comment' as their comment node name, but if they don't we
+        // can still detect comments by looking at their syntax
+        // highlighting.
         AtomKind::Comment
     } else if highlights.keyword_ids.contains(&node.id()) {
         AtomKind::Keyword