Added before_filter to Gtpay::TransactionsController#callback to fail early when transaction is not pending
Using Api Response to set GtpayTransaction#status instead of response hash
Removed message returning logic based on Api Response from GtpayTransaction Model
Finding string message based on GtpayTransaction#status in controller
Raising exception when gtpay_merc_id is set nil as gateway is not reachable w/o it