Add helper for getting error messages from .fail()

[JTimings]

Is your feature request related to a problem? Please describe.

Prior to v5 we could get the specific error message returned by Spree using response.fail().serverResponse.data.error. This no longer seems possible, or at least an alternative not documented.

Describe the solution you'd like.

Would like to return more detailed error messages to users upon response.fail()

Additional context

We're using JavaScript for the frontend and backend v4.3.0

[ferentc]

Any updates on this? have same issue.