Change form field error reporting.

This PR changes the structure of validation error custom events on form fields. This is necessary because currently the event action is the pathname. The action is what gets reported to Optimizely thus we can't track error on individual form fields.

The new structure is this:
category: 'form error'
action: 'email error'
label: pathname

Only the action gets reported to platforms that don't have structured data. So with this change, "email error" is what gets reported to Optimizely.