improve the error message for a dropped current role

mysql-test/suite/roles/drop_current_role.result

@@ -2,4 +2,4 @@ create role r;
 set role r;
 drop role r;
 revoke all on *.* from current_role;
-ERROR 42000: There is no such grant defined for user 'r' on host ''
+ERROR OP000: Invalid role specification `r`.

mysql-test/suite/roles/drop_current_role.test

@@ -5,5 +5,5 @@
 create role r;
 set role r;
 drop role r;
-error ER_NONEXISTING_GRANT;
+error ER_INVALID_ROLE;
 revoke all on *.* from current_role;

sql/sql_acl.cc

@@ -3475,13 +3475,15 @@ static int replace_user_table(THD *thd, TABLE *table, LEX_USER &combo,
            table->key_info->key_length);
 
   if (table->file->ha_index_read_idx_map(table->record[0], 0, user_key,
-                                         HA_WHOLE_KEY,
-                                         HA_READ_KEY_EXACT))
+                                         HA_WHOLE_KEY, HA_READ_KEY_EXACT))
   {
     /* what == 'N' means revoke */
     if (what == 'N')
     {
-      my_error(ER_NONEXISTING_GRANT, MYF(0), combo.user.str, combo.host.str);
+      if (combo.host.length)
+        my_error(ER_NONEXISTING_GRANT, MYF(0), combo.user.str, combo.host.str);
+      else
+        my_error(ER_INVALID_ROLE, MYF(0), combo.user.str);
       goto end;
     }
     /*