Improve UI for unpacked extensions failing to load

chrome/browser/resources/extensions/extensions.css

 /* Copyright (c) 2012 The Chromium Authors. All rights reserved.
  * Use of this source code is governed by a BSD-style license that can be
  * found in the LICENSE file. */
 
 html.loading * {
   -webkit-transition-duration: 0 !important;
 }
 
 html:not(.focus-outline-visible)
 :enabled:focus:-webkit-any(input[type='checkbox'], input[type='radio']) {
@@ skipping 82 matching lines @@
 
 #dev-controls-spacer {
   -webkit-box-flex: 1;
 }
 
 #dev-toggle {
   margin-top: 0;
   text-align: end;
 }
 
+.extension-code {
+  border: 1px solid #ccc;
+  display: flex;
+  font-family: monospace;
+  overflow: auto;
+  white-space: pre;
+}
+
+.extension-code > * {
+  padding: 3px;
+}
+
+.extension-code-line-numbers {
+  align-self: flex-start;
+  background-color: rgba(240, 240, 240, 1);
+  border-right: 1px solid #ccc;
+  color: rgba(128, 128, 128, 1);
+  flex-shrink: 0;
+  text-align: right;
+}
+
 #extension-settings:not(.dev-mode) .developer-extras {
   display: none;
 }
 
 .developer-extras > div,
 .permanent-warnings > div {
   margin: 5px 0;
 }
 
 #extension-settings #page-header {
@@ skipping 253 matching lines @@
 /* Sideload Wipeout */
 
 .sideload-wipeout-learn-more {
   text-decoration: none;
 }
 
 .sideload-wipeout-banner .page-banner-text {
   -webkit-padding-start: 8px;
   background-image: none;
 }