Improve async store error when sync api used

[KazuhiroItoh]

• [x] Bugfix (non-breaking change which fixes an issue)
• [ ] New feature (non-breaking change which adds functionality)

Proposed changes

Improved the error message when callback is not specified in get() to an appropriate message.

This is a fix for the same issue pointed out in the PouchDB Context store plugin. https://github.com/node-red/node-red-context-pouchdb/pull/1#issuecomment-866750901

Checklist

• [x] I have read the contribution guidelines
• [ ] For non-bugfix PRs, I have discussed this change on the forum/slack team.
• [ ] I have run grunt to verify the unit tests pass
• [ ] I have added suitable unit tests to cover the new/changed functionality