Localize/reword "Loader not implemented" error message

app/src/main/java/eu/kanade/tachiyomi/ui/reader/loader/ChapterLoader.kt

@@ -87,7 +87,7 @@ class ChapterLoader(
                     is LocalSource.Format.Epub -> EpubPageLoader(format.file)
                 }
             }
-            else -> error("Loader not implemented")
+            else -> error(context.getString(R.string.loader_not_implemented_error))
         }
     }
 }

app/src/main/res/values/strings.xml

@@ -560,6 +560,7 @@
     <string name="transition_pages_loading">Loading pages…</string>
     <string name="transition_pages_error">Failed to load pages: %1$s</string>
     <string name="page_list_empty_error">No pages found</string>
+    <string name="loader_not_implemented_error">Source not found</string>
 
     <!-- Updates fragment -->
     <string name="updating_library">Updating library</string>