Adjust handling of enrollment when customer already exists in group

[thekaveman]

Our payment processor uses a 500 error response to indicate that a customer is already enrolled in a group.

This is not an actual problem for Benefits, so we have code that handles the response and returns a success to the user.

The error handling is brittle, relying to some degree on the returned error message. This message has now changed, thus our handling is broken.

To Reproduce

Steps to reproduce the behavior:

1. Go to test
2. Verify as a senior with SacRT
3. Enroll a card
4. Do 2-3 again with the same card --> BOOM

Expected behavior

Attempting to re-enroll with the same card should not cause an error for the end user.

Additional context

New error message from the logs looks like:

Customer xxxx already exists in group yyyy

The old one looked more like:

Duplicate customer xxxxx...

[cal-itp-sentry]

Sentry issue: BENEFITS-48