Return better errors from the router bridge

[bnjjj]

related to https://github.com/apollographql/federation-rs/pull/119

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[github-actions[bot]]

@bnjjj your pull request is missing a changelog!

[abernix]

We can easily re-open this and restore the branch, if we find we need it in the future, but at this point I think this has been open for a bit too long.