uploads: add column for error messages.

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

db/migrate/20220207195123_add_error_to_uploads.rb

@@ -0,0 +1,6 @@
+class AddErrorToUploads < ActiveRecord::Migration[7.0]
+  def change
+    add_column :uploads, :error, :text
+    add_index :uploads, :error, where: "error IS NOT NULL"
+  end
+end