Coverage failure should be more explicit

[Aaron Gunderson]

We are running elixir:latest in CI and hit this ambiguous problem with the new change to coverage thresholds reporting an error:

Changelog notes:

[mix test] Exit with a status of 3 when coverage falls below threshold

Reading CI it was unclear what was failing as the only indication of an error is the red on the coverage percentage which obviously isn't visible in this CI configuration. I think it would be much better to have an explicit failure line like:

```

-----------|--------------------------
    63.13% | Total
    
Coverage failed, threshold not met:
  Coverage:  63.13%
  Threshold: 75%
 
See https://hexdocs.pm/mix/1.13.0/Mix.Tasks.Test.html#module-coverage

Generated HTML coverage results in "cover" directory
```

Thoughts?

[José Valim]

I like the additional input on why it failed, a PR is definitely welcome.

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[Aaron Gunderson]

Thank you José!

For context closure we resolved this in https://github.com/elixir-lang/elixir/pull/11457