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[bot]]

Coverage report

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

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[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.