chore(integrations): Changes exception type for missing org integrations, updates assignment sync to halt on this exception

[GabeVillalobos]

Fixes ECO-507

Updates the exception type raised in the base IntegrationInstallation class to a distinct type, handles this case when choosing an SLO outcome in sync_assignment_outbound tasks to ensure that missing org integrations are treated as halts, not failures.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

✅ All tests successful. No failed tests found.

Additional details and impacted files

@@             Coverage Diff             @@
##           master   #91542       +/-   ##
===========================================
+ Coverage   45.40%   87.64%   +42.23%     
===========================================
  Files       10299    10321       +22     
  Lines      585388   586474     +1086     
  Branches    22602    22602               
===========================================
+ Hits       265804   514013   +248209     
+ Misses     319156    72033   -247123     
  Partials      428      428               

[sentry-io]

Suspect Issues

This pull request was deployed and Sentry observed the following issues:

• ‼️ OrganizationIntegrationNotFound: missing org_integration sentry.integrations.tasks.sync_status_outbound View Issue
• ‼️ OrganizationIntegrationNotFound: missing org_integration sentry.tasks.process_commit_context View Issue
• ‼️ OrganizationIntegrationNotFound: missing org_integration sentry.integrations.tasks.sync_status_outbound View Issue
• ‼️ OrganizationIntegrationNotFound: missing org_integration sentry.tasks.process_commit_context View Issue
• ‼️ OrganizationIntegrationNotFound: missing org_integration /api/0/projects/{organization_id_or_slug}/{proj... View Issue

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