Improve error message when comparing non-comparable types

[tony-ho]

Fixes #27910

[msftclas]

All CLA requirements met.

[RyanCavanaugh]

@DanielRosenwasser are you good with this verbiage as-is?

[RyanCavanaugh]

I'll rebase this. We also decided to not explicitly recommend unknown since a tighter lower bound (such as string or boolean) is often more appropriate.

[sandersn]

@RyanCavanaugh do you just want that sentence removed from the error?

[sandersn]

Unfortunately, we never finished reviewing this PR. It is pretty old now, so I'm going to close it to reduce the number of open PRs.