avoid extra redirect; correctly show auth.net errors on failed transaction

This fixes #30. It avoids an extra redirect, and in so doing preserves the error messages for the end user.

One day we should show them via SweetAlert...

mentioned in merge request !43 (closed)

mentioned in issue authnet#30 (closed)

added 5 commits

• 6ff8bce5...c826ca8c - 4 commits from branch extensions:master
• b0c99a33 - avoid extra redirect; correctly show auth.net errors on failed transaction

Compare with previous version

merged