Fix backtrace display in error page

Author: noteflakes

examples/bad.rb

@@ -0,0 +1,9 @@
+export template {
+  div {
+    markdown <<~MD
+      ```ruby
+      @buffer << #{foo}
+      ```
+    MD
+  }
+}.tap { puts '!' * 40; puts Papercraft.compiled_code(it.proc); puts}

lib/syntropy/applets/builtin/default_error_handler.rb

@@ -30,8 +30,12 @@
 
 def transform_backtrace(backtrace)
   backtrace.map do
-    location = it.match(/^(.+:\d+):/)[1]
-    { entry: it, url: "vscode://file/#{location}" }
+    if (m = it.match(/^(.+:\d+):/))
+      location = m[1]
+      { entry: it, url: "vscode://file/#{location}" }
+    else
+      { entry: it, url: nil }
+    end
   end
 end