Improve error message when testing on not-yet-published crate

[obi1kenobi]

Right now, if a user runs cargo-semver-checks on a new package that hasn't been uploaded to crates.io, they'll get a fairly opaque error message saying the crate was "not found in registry."

This is a common failure mode for first-time users of the tool, since most linters only depend on the current state of the repo instead of also depending on prior versions.

We should consider adding hint/info text reminding the user that the default mode assumes there's a prior version on crates.io and suggesting other baseline options if that's not the case.