Standardize the way we check for and display form errors

• Some views had a "Close" button. We've removed this, because we don't want users accidentally hiding the validation errors and not knowing what needs to be fixed.
• Some views used li, some used p, some used span. We've standardized on li.
• Some views only showed the first error. We've standardized on showing all of them.
• Some views added an #error_explanation div, which we've made standard.