RS Post Settings: Use consistent error message in fetchPost

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

Author: nbradbury

WordPress/src/main/java/org/wordpress/android/ui/postsrs/PostRsSettingsViewModel.kt

@@ -825,7 +825,7 @@ class PostRsSettingsViewModel @Inject constructor(
     private suspend fun fetchPost():
         AnyPostWithEditContext = withContext(Dispatchers.IO) {
         val client = apiClient
-            ?: throw PostApiRequestException("No API client")
+            ?: throw PostApiRequestException("No site selected")
         val response = client.request {
             it.posts().retrieveWithEditContext(
                 PostEndpointType.Posts,