[2.x] Update Cashier::api error message

[Ahmad-Alkaf]

Hello, I just spent two days debugging an error that can be easily debugged, if the error message is descriptive. But Paddle seems just to say "Invalid Request" or this is what I thought. My problem was not a bug it was a "bad request" because I did not pass something important (email) that my Billable class didn't have (Team). Nevertheless, the way I debugged this, is by digging to the source of the error (Cashier::api) and dd its response which is the following:

Clearly, the error by Paddle is descriptive but its (detail) not very much. I don't want anyone to waste his time debugging an error that does not exist on the error stack. Before my pull request:

After: Thank you very much for your patience, keep up the great work!

[driesvints]

Thanks @Ahmad-Alkaf. I changed the message format here a bit and added an extra safeguard when creating customers.