CHI-3687: Fix exception handling for getTaskAndReservations

[stephenhand]

Description

Make the endpoints related to task assignment return a 404 rather than a 500 when the task cannot be found

This will allow the Flex UI to gracefully handle this scenario and continue on to finalizing the contact

Checklist

• Corresponding issue has been opened
• New tests added
• [n/a] Feature flags added
• [n/a] Strings are localized
• Tested for chat contacts
• Tested for call contacts

Other Related Issues

None

Verification steps

AFTER YOU MERGE

1. Cut a release tag using the Github workflow. Wait for it to complete and notify in the #aselo-deploys Slack channel.
2. Comment on the ticket with the release tag version AND any additional instructions required to configure an environment to test the changes.
3. Only then move the ticket into the QA column in JIRA

You are responsible for ensuring the above steps are completed. If you move a ticket into QA without advising what version to test, the QA team will assume the latest tag has the changes. If it does not, the following confusion is on you! :-P