Fix CheckWidths error message for uninferred width (backport #2196)

[mergify]

This is an automatic backport of pull request #2196 done by Mergify.

Cherry-pick of adc2ad9 has failed:

On branch mergify/bp/1.3.x/pr-2196
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	both modified:   src/main/scala/firrtl/passes/CheckWidths.scala

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