fix for bug #12591 (SHOW TABLES FROM dbname produces wrong error message)

mysql-test/r/show_check.result:
  results for test for bug #12591
mysql-test/t/show_check.test:
  test for bug #12591
sql/sql_show.cc:
  don't let my_dir() emit a message bug catch the errno (my_errno - a thread specific variable) and check it for ENOENT - not existing.
  For other errors emulate the message used to be emitted by my_dir()

mysql-test/r/show_check.result

@@ -555,3 +555,5 @@ Database	Table	In_use	Name_locked
 DROP TABLE txt1;
 DROP TABLE tyt2;
 DROP TABLE urkunde;
+SHOW TABLES FROM non_existing_database;
+ERROR 42000: Unknown database 'non_existing_database'

mysql-test/t/show_check.test

@@ -405,3 +405,8 @@ SHOW OPEN TABLES;
 DROP TABLE txt1;
 DROP TABLE tyt2;
 DROP TABLE urkunde;
+#
+# BUG #12591 (SHOW TABLES FROM dbname produces wrong error message)
+#
+--error 1049
+SHOW TABLES FROM non_existing_database;

sql/sql_show.cc

@@ -264,8 +264,14 @@ mysql_find_files(THD *thd,List<char> *files, const char *db,const char *path,
 
   bzero((char*) &table_list,sizeof(table_list));
 
-  if (!(dirp = my_dir(path,MYF(MY_WME | (dir ? MY_WANT_STAT : 0)))))
+  if (!(dirp = my_dir(path,MYF(dir ? MY_WANT_STAT : 0))))
+  {
+    if (my_errno == ENOENT)
+      my_error(ER_BAD_DB_ERROR, MYF(ME_BELL+ME_WAITTANG), db);
+    else
+      my_error(ER_CANT_READ_DIR, MYF(ME_BELL+ME_WAITTANG), path, my_errno);
     DBUG_RETURN(-1);
+  }
 
   for (i=0 ; i < (uint) dirp->number_off_files  ; i++)
   {