Fix: throw error message instead of the error object when submitting an donation made with Stripe Element gateway

[glaubersilva]

Resolves GIVE-345

Description

This PR fixes the error message that was being displayed in certain cases as [object Object] when a donation made with Stripe Element gateway was submitted. Now we are displaying a proper string to indicate the error instead of an object.

Affects

The Stripe Element gateway

Visuals

Testing Instructions

Credit Card:

1. Create a donation and miss any of CC #, expiry, or CVV and press Donate;
2. The new friendly error message will show above the Donate button.

Google Pay:

1. Enable Google Pay in your Stripe Dashboard;
2. Create a donation and press Donate, a modal should be opened;
3. Close the Google modal;
4. The new friendly error message will show above the Donate button.

To set up Google Pay in your Chrome for Desktop, check out this link: https://givewp.com/documentation/add-ons/stripe-gateway/stripe-apple-pay-google-pay/

Pre-review Checklist

• Acceptance criteria satisfied and marked in related issue
• Relevant @unreleased tags included in DocBlocks
• Includes unit tests
• Reviewed by the designer (if follows a design)
• Self Review of code and UX completed

[glaubersilva]

@jonwaldstein This is ready for re-review.

[jonwaldstein]

Thanks for the updates, looks good 👍

[rickalday]

Passed manual QA tests.