improved error message when committing without a name or email set.

[zennetizen]

More granular error reporting for when a user or email is not provided.

[CLAassistant]

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[VinaiRachakonda]

Thanks for your contribution @zennetizen! I will look at your pr this weekend, add some test cases, and merge it in.

[VinaiRachakonda]

Hi @zennetizen. Can you copy over the changes in this branch: https://github.com/dolthub/dolt/tree/pr/1431 and update your pr? For some reason I can't push changes directly to your pr.