Fix/manga white screen missing extension (#285)

* Prevent white screen when source is missing

A undefined source caused a TypeError

* Show error message when source is missing

src/i18n/locale/en.json

@@ -238,7 +238,8 @@
       "filter": "Filter",
       "loading": "Loading...",
       "none": "None",
-      "sort": "Sort"
+      "sort": "Sort",
+      "unknown": "Unknown"
     },
     "language": {
       "label": {
@@ -435,7 +436,8 @@
     },
     "error": {
       "label": {
-        "no_sources_found": "No sources found. Install Some Extensions first."
+        "no_sources_found": "No sources found. Install Some Extensions first.",
+        "source_not_found": "Could not find source. Check your installed extensions."
       }
     },
     "local_source": {