fix: Unparseable config when using the low-performance profile (#1570)

* The `low-performance` profile itself had parse errors: fix them!
* Do not silently swallow override parse errors, they are always a user issue
* Minor error message rendering improvements to show multiple pieces of context

src/scss/message.scss

@@ -58,4 +58,6 @@ div.message div.icon {
 .message pre {
   margin-top: 20px;
   overflow-x: auto;
+  border: 1px dotted var(--divider-color);
+  padding: 1em;
 }