MDEV-21586 Server does not start if lc_messages setting was not english.

Fixed a bug introduced in  MDEV-11345, server did not start if
non-english error messages were set in startup parameters.

Added lc_messages=de_DE option into an existing test case.

mysql-test/r/locale.result

@@ -51,7 +51,7 @@ DROP TABLE t1;
 #
 SET lc_messages=sr_YU;
 Warnings:
-Warning	1287	'sr_YU' is deprecated and will be removed in a future release. Please use sr_RS instead
+Warning	1287	'sr_YU' ist veraltet. Bitte benutzen Sie 'sr_RS'
 SHOW VARIABLES LIKE 'lc_messages';
 Variable_name	Value
 lc_messages	sr_RS

mysql-test/t/locale.opt

@@ -0,0 +1 @@
+--lc-messages=de_DE

sql/derror.cc

@@ -84,8 +84,9 @@ bool init_errmessage(void)
   if (!use_english)
   {
     /* Read messages from file. */
-    use_english= !read_texts(ERRMSG_FILE,lang, &original_error_messages);
-    error= TRUE;
+    error= use_english= read_texts(ERRMSG_FILE,lang, &original_error_messages);
+    if(error)
+      sql_print_error("Could not load error messages for %s",lang);
   }
 
   if (use_english)