Description
Is your feature request related to a problem? Please describe.
Scripted custom fields that don't appear in or match exactly with the loaded CSV will show as plain text (e.g. {polling_location}) rather than populating. This can be caught by looking for custom fields that appear as red rather than green, but the campaign can still be started and texts like, "Will you donate {ask} today?" can still be sent.
Describe the solution you'd like
When the start button is used on a campaign with invalid custom fields, a warning box should prompt the user to review the script and starting the campaign should be disallowed. With the campaign unstarted, the user can either reupload a new csv or fix/omit the invalid custom field within the script.
Describe alternatives you've considered
Invalid custom fields could not populate but also not appear, resulting in texts like, "Will you donate today?" but also texts like "Will you join us at the on at ?"
Additional context
