fix: fallback to 'price' in ecommerce api loader

[tecoholic]

Description

The Django Oscar upgrade of ecommerce changed the
item's price field from price_excl_tax to just price
causing the EcommerceApi data loader to fail.

This commit handled the exception and falls back to
the 'price' value.

Ref: openedx-unsupported/ecommerce#4050

Additional information

The same as openedx#4291.

Testing instructions

Reproduce Issue

1. Setup Tutor 18 (Redwood) dev env with the plugins ecommerce and discovery enabled
2. Clone course-discovery.
3. Add a tutor mount for the course-discovery.
4. Create a course or import the demo course.
5. Setup admin user for ecommerce and discovery
6. Add a course seat in http://ecommerce.local.edly.io:8130/courses/ for the test course
7. Run the following from Discovery Shell to set the ecommerce API url. NOTE: trying the same in Discovery admin fails due to URL validation.

   from course_discovery.apps.core.models import Partner
   p = Partner.objects.get(short_code="dev")
   p.ecommerce_api_url = "http://ecommerce:8130/api/v2/"
   p.save()
   

8. Run ./manage.py refresh_course_metadata --partner dev. It should fail with the KeyError price_excl_tax.

Testing the fix from PR

1. Checkout the PR branch
2. Run ./manage.py refresh_course_metadata --partner dev again. This time, it should run successfully.

[Agrendalath]

@tecoholic, you can apply the suggestion from the upstream PR if you'd like.

👍

• I tested this: checked the related changes
• I read through the code
• I checked for accessibility issues: n/a
• Includes documentation: n/a
• Added to the Code Drift project board (for backports): upstream PR already present: fix: use "price" in ecommerce data loader openedx/course-discovery#4450

[tecoholic]

@Agrendalath Thanks for the review. I applied the suggestion from upstream reviewer.