Show user facing errors for CustomersDot API calls

What does this MR do?

Show "user facing" errors (feedback) for CustomersDot API calls

Related to #330603 (closed)

1. Modified the standard fallback message by removing "We encountered an error and " which was duplicated across many screens which already have an intro like "We have found the following errors: "

• BEFORE: "We have found the following errors: We encountered an error and our team has been notified. Please try again."
• AFTER: "We have found the following errors: our team has been notified. Please try again."

2. We now let "[data][errors]" which come from the CustomersDot to be shown. In a previous MR !59487 (merged) we hid them due to leaking credentials, which isn't the case anymore due to changes in the CustomersDot probably.

After a thorough investigation it turns out the JSON output for does contain information useful to the end user: like Zuora messages, or custom user facing messages.

{:success=>false,  :data=>{ :errors=>"You have insufficient funds." }}

BEFORE: "We have found the following errors: We encountered an error and our team has been notified. Please try again."

EXAMPLES AFTER:

1. We have found the following errors: HTTP status code: 502

2. We have found the following errors: PG::StringDataRightTruncation: ERROR: value too long for type character varying(40)

3. We have found the following errors: undefined method `merge' for nil:NilClass

4. [BusinessValidationError] Expiration date must be a future date.

5. Failed to confirm your order: "HTTP status code: 500". Please try again.

6. Failed to confirm your order: Your card has insufficient funds.

7. All the other zuora errors #330603 (closed)

8. We still catch the HTTP 500 errors, which usually contain non-business errors.

See a list of example errors tracked in sentry

Does this MR meet the acceptance criteria?

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Security

Does this MR contain changes to processing or storing of credentials or tokens, authorization and authentication methods or other items described in the security review guidelines? If not, then delete this Security section.

• Label as security and @ mention @gitlab-com/gl-security/appsec
• The MR includes necessary changes to maintain consistency between UI, API, email, or other methods
• Security reports checked/validated by a reviewer from the AppSec team