Fail the check if plugin is enabled but no artifacts are set

[2m]

Currently if mimaPreviousArtifacts setting is empty the mimaReportBinaryIssues task succeeds without running any checks. Instead this task should fail with an error message suggesting to set artifacts to check binary compatibility against.

If a sub-module does not wish its BC to be checked, it should disable MiMa autoplugin altogether with .disablePlugins(MimaPlugin).

[dwijnand]

This is a semantic breaking change, but I think it's a good one we should do, with some escape routes.

The first is disabling the plugin. But additionally, given it's not possible to disable a plugin build-wide (indeed you can't make any local build-wide decisions about plugins 😕) I think, for those complex builds out there, we should provide a build-level key that basically reverts this change. An opt-out, basically.

[dwijnand]

In the case of cross-building to a new scala major version (e.g 2.13) today you define mimaPreviousArtifacts to be the empty set when building 2.13, with this change you toggle the key I was describing above.

[dwijnand]

Asking twitter: https://twitter.com/dwijnand/status/1059860677630812160