After #2088, we began calling `Diff` on providers that are not configured
due to unknown configuration values. This hit an assertion intended to
detect exactly this scenario, which was previously unexpected.
These changes adjust `Diff` to indicate that a Diff is unavailable and
return an error message that describes why. The step generator then
interprets the diff as indicating a normal update and issues the error
message to the diagnostic stream.
Fixes#2223.
ab36b1116f