refactor: set_as_failed method on Payment Request

[batonac]

This PR removes the set_failed method from the Payment Request Doctype, which did nothing and was referenced nowhere.

It replaces the set_failed method with a set_as_failed method which corresponds generally to the existing set_as_cancelled and set_as_paid methods, with the notable difference that it accepts a reason parameter which populates the recently-introduced failed_reason field.

This is somewhat related to my recent PR#75 on the payments app which attempts to incorporate payment status from GoCardless webhooks, including updating the failed_reason field, where applicable. In that PR I utilize the existing set_as_cancelled and set_as_paid methods for those actions, but have created a manual workaround for the absence of a set_as_failed method. It would be nice to have this consolidated eventually.

[stale]

This pull request has been automatically marked as inactive because it has not had recent activity. It will be closed within 3 days if no further activity occurs, but it only takes a comment to keep a contribution alive :) Also, even if it is closed, you can always reopen the PR when you're ready. Thank you for contributing.

[batonac]

Keep alive ~ Any feedback would be appreciated!

[batonac]

BTW, here's a quick verification that the set_failed method had no references anywhere else:

[stale]

This pull request has been automatically marked as inactive because it has not had recent activity. It will be closed within 3 days if no further activity occurs, but it only takes a comment to keep a contribution alive :) Also, even if it is closed, you can always reopen the PR when you're ready. Thank you for contributing.