Add config to customize the default error message

Add a lowlevel_error_handler, so we can customize the default error
message.

example:

```
lowlevel_error_handler do
  [302, {'Content-Type' => 'text', 'Location' => 'foo.html'}, ['302 found']]
end
```

[fix #458]
Merge pull request #470 from arthurnn/fix_458

Add config to customize the default error message