Cleanup in the system variable parsing code

- Adding "return true" into LEX::set_system_variable()
  and LEX::set_default_system_variable() after my_error().
  This makes the parser exit on error immediately.
  Previously, the error was caught only in mysql_parser(),
  a few lines after the parse_sql() call.
- Fixing "--error 1272" to "--error ER_VARIABLE_IS_NOT_STRUCT" in tests

mysql-test/main/key_cache.test

@@ -44,7 +44,7 @@ SET @@global.key_buffer_size=@save_key_buffer_size;
 
 --error 1064
 SELECT @@default.key_buffer_size;
---error 1272
+--error ER_VARIABLE_IS_NOT_STRUCT
 SELECT @@skr.storage_engine="test";
 
 select @@keycache1.key_cache_block_size;

sql/sql_lex.cc

@@ -7011,7 +7011,10 @@ bool LEX::set_default_system_variable(enum_var_type var_type,
   if (!var)
     return true;
   if (!var->is_struct())
+  {
     my_error(ER_VARIABLE_IS_NOT_STRUCT, MYF(0), name->str);
+    return true;
+  }
   return set_system_variable(var_type, var, &default_base_name, val);
 }
 
@@ -7040,7 +7043,10 @@ bool LEX::set_system_variable(THD *thd, enum_var_type var_type,
     return true;
   }
   if (!tmp->is_struct())
+  {
     my_error(ER_VARIABLE_IS_NOT_STRUCT, MYF(0), name2->str);
+    return true;
+  }
   return set_system_variable(var_type, tmp, name1, val);
 }