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.
See merge request !3531
