Feat: system enrollment error - 500 error when linking

[angela-tran]

Part of #2032

This PR implements the system enrollment error page template and adds error-handling to the benefits.enrollment.views.index function so that if a 500-level HTTP error occurs when we try to link a funding source to a group, the system enrollment error page will be shown. The CTA takes the user back to the agency index.

A Sentry notification will be sent - see an example from my testing. Note: since I don't know how to cause an actual 500 error with the linking endpoint, I had to add some code locally to fake it (more details in steps for reviewing / testing).

Screenshots

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Desktop	Mobile

Steps for reviewing / testing

To simulate a 500-level HTTP error, I added this to my index function:

       try:
+           class response:
+               def __init__(self):
+                   self.status_code = 500
+           raise HTTPError(response=response())
            client.link_concession_group_funding_source(funding_source_id=funding_source.id, group_id=eligibility.group_id)
        except HTTPError as e:

Then run through a Login.gov flow and non-Login.gov flow all the way through enrollment as normal to see the system enrollment error page.

[github-actions]

Coverage report

Click to see where and how coverage changed

File	Statements	Missing	Coverage	Coverage (new stmts)	Lines missing
benefits/enrollment
views.py
Project Total

This report was generated by python-coverage-comment-action

[angela-tran]

I forgot to run ./bin/makemessages.sh! Commit incoming

[thekaveman]

I rebased on dev to clean up conflicts in the PO files

[thekaveman]

Rebased on dev one more time to resolve conflicts with #2067, and added the Spanish translations for this page.

I also updated the page title in English and Spanish using the latest from the copy spreadsheet.

[thekaveman]

This looks good!

I was able to trigger the error using the suggested code edit, and then confirmed that the Sentry and Amplitude events were sent, copy is correct, and the CTA takes the user back to the agency homepage.