Removing the margin-bottom from select.error

There's currently an inconsistency in the way error classes are handled:

When a `select` is within an element with a class of 'error', they are treated the same as other inputs:
(Line 58)
```css
.error {
      input,
      textarea,
      select {
        margin-bottom: 0;
      }
```

...but when the error class is on the element itself, only inputs and textareas are styled. This can lead to a gap being displayed between a select and it's error message.