uploads: add column for error messages.

Change it so uploads store errors in an `error` column instead of in the
`status` field.

script/fixes/100_migrate_upload_errors.rb

@@ -0,0 +1,11 @@
+#!/usr/bin/env ruby
+
+require_relative "base"
+
+with_confirmation do
+  Upload.where("status ~ '^error:'").find_each do |upload|
+    message = upload.status.delete_prefix("error: ").strip
+    upload.update_columns(status: "error", error: message)
+    puts({ id: upload.id, status: "error", error: message }.to_json)
+  end
+end