paypro: fix xhr error messages. fix clearing $root.merchant on section switch. check for merchant_data.

This fixes the cross-origin error messages. XHR is kind of weird about this: it doesn't generate its own error message, so we have to examine the XHR object and create one.

UPDATE: This also fixes the clearing of $root.merchant when switching scopes/tabs/pages/controllers/views/section/whateverelseyoucancallit.

UPDATE: This also adds a check for merchant_data if the server doesn't send it. (It's optional).