Show error when trying to download chapters from not installed source (closes #4283)

app/src/main/java/eu/kanade/tachiyomi/source/SourceManager.kt

@@ -49,7 +49,7 @@ open class SourceManager(private val context: Context) {
         LocalSource(context)
     )
 
-    private inner class StubSource(override val id: Long) : Source {
+    inner class StubSource(override val id: Long) : Source {
 
         override val name: String
             get() = id.toString()

app/src/main/java/eu/kanade/tachiyomi/ui/manga/MangaController.kt

@@ -917,6 +917,11 @@ class MangaController :
     }
 
     private fun downloadChapters(chapters: List<ChapterItem>) {
+        if (source is SourceManager.StubSource) {
+            activity?.toast(R.string.loader_not_implemented_error)
+            return
+        }
+
         val view = view
         val manga = presenter.manga
         presenter.downloadChapters(chapters)