Proper boostrap specific error message styling on forms

[anselmdk]

This is not super clean but it allows you to easily use the following in your forms:

$this->sessionMessage("my message", 'bad'); //could be good, bad, or warning

See http://api.silverstripe.org/3.1/source-class-Form.html#1052-1067

I also removed the alert-error style - don't see any need for it -maybe it's a leftover from Bootstrap 2.