Better error message for unplanned components

[hamishmack]

While using cabal-to-nix to replace a package in an existing project @shlevy and I had an issue where we got an error similar to #1372. It turned out that we had not set the .components.libarary.planned and as a result packages that depended on it could not find the library.

The error message we got was very unhelpful and I think it should be possible (perhaps by including a simple stub in place of the missing component) to have a message that said something like "The component foo is not available because it was not in the plan".

[stale[bot]]

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