Gracefully handle errors from the `datastore.put` response

[zimeg]

Type of change

• [ ] New sample
• [x] New feature
• [ ] Bug fix
• [ ] Documentation

Summary

This PR adds error handling to the datastore.put call, returning an error if this call is unsuccessful. Related tests are also added.

Unsure if this is the ideal way to mock different responses or if this is how we'd like to handle this error case, so please feel free to suggest alternatives!

Notes

Mocked responses were updated to match https://api.slack.com/methods/apps.datastore.put#examples

Requirements

• [x] I’ve checked my submission against the Samples Checklist to ensure it complies with all standards
• [x] I have ensured the changes I am contributing align with existing patterns and have tested and linted my code
• [x] I've read and agree to the Code of Conduct

[zimeg]

@hello-ashleyintech no worries at all! The LGTM is always appreciated 🙌