Early out and report error if langauge files are missing

This is a common problem if you don't set up your paths properly on
Linux, so be nice and try to provide helpful message why we failed to
load instead of crashing on user a moment later.

src/localisation/string_ids.h

@@ -2146,6 +2146,7 @@ enum {
 	STR_STAY_CONNECTED_AFTER_DESYNC = 5557,
 
     STR_RESTART_REQUIRED = 5558,
+    STR_LANGUAGE_LOAD_FAILED = 5561,
 
 
 	// Have to include resource strings (from scenarios and objects) for the time being now that language is partially working