Provide guidance links in mix tasks' Logger.error output

[vanderhoop]

We just added excellent_migrations to a large repository with many newish Elixir developers contributing to it. I noticed that the current Logger.error output can be somewhat cryptic, and it isn't clear what the next steps are or where the errors originate.

This pull request adds links to the README's 'Checks' and 'Assuring Safety' section to the error output of the mix tasks.

I haven't yet determined how to do the same for the credo check, but I'll happily add it if this first PR is welcomed.

Before

After

[vanderhoop]

The link fragments here are inherently brittle, but if the README headings change without the appropriate fragment updates here, the user will still end-up on the README document, which is still a net UX win.

[Artur-Sulej]

Thanks for the PR and using the lib!
I like your solution, because it quickly adds value and I'll be happy to merge your PR after testing.

I was thinking about introducing something similar myself. My idea was to add README links to specific dangers that were detected. Additionally these links would point to README from a lib version that is used.

For example column_renamed check is triggered when using excellent_migrations 0.1.6 – and the message would contain this link: https://github.com/Artur-Sulej/excellent_migrations/blob/v0.1.6/README.md#renaming-a-column
With this approach users are pointed to the right place in docs and links would always work even if changed in newer versions.

After adjusting links to match check names, links could be built with this logic:

def build_link(current_version, anchor) do
  anchor =
    check
    |> Atom.to_astring()
    |> String.replace("_", "-")

  "#{@base_url}/blob/v#{current_version}/README.md##{anchor}"
end

Anyway, that could be done in the future.

[vanderhoop]

After adjusting links to match check names, links could be built with this logic:

def build_link(current_version, anchor) do
  anchor =
    check
    |> Atom.to_astring()
    |> String.replace("_", "-")

  "#{@base_url}/blob/v#{current_version}/README.md##{anchor}"
end

I actually started down a path of trying to build out links and aborted due to not knowing if you'd be interested in a bigger change, but I hadn't considered tying the links to the particular release tag to avoid brittleness. That's 💯

Happy to take on the subsequent change once this first one goes in.

[vanderhoop]

@Artur-Sulej I'm curious how you're feeling about this. I just got around to auditing my dependencies on a side project, and realized my fork was behind origin. These warnings have aided me personally :)