Warnings about missing Cargo.toml keys are scarier than need be

[fitzgen]

When watching a stream of someone going through our tutorial, they got surprised/flustered/worried by these warning, so I think we can tone it down a bit somehow.

Perhaps this can just be "info" instead of all-caps and all-bold WARN? Perhaps we can collapse all the missing keys warnings into a single warning instead of multiple?

[mstallmo]

I can get working on this if no one else has already claimed it!