Handle attempt to use non-existing resolver
Handle attempt to use non-existing resolver

The old behaviour is described in #434

The new behavior looks as follows:

    Downloading lts-42.0 build plan ...SnapshotNotFound "lts-42.0"
    Non existing resolver: "lts-42.0".
    For a complete list of available snapshots see https://www.stackage.org/snapshots

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