fix: improve error message in SPA mode (#1973)

Author: kermanx

Diff for: packages/client/layouts/error.vue

@@ -1,5 +1,9 @@
 <template>
   <div class="px-4 py-10 text-center text-red-700 dark:text-red-500 font-bold font-mono">
-    An error occurred on this slide. Check the terminal for more information.
+    {{
+      __SLIDEV_HAS_SERVER__
+        ? 'An error occurred on this slide. Check the terminal for more information.'
+        : 'Failed to fetch this slide. Please check your network connection.'
+    }}
   </div>
 </template>