Print the optional Printable passed to stop ops

[nandor]

Contributor Checklist

• Did you add Scaladoc to every public function/method?
• Did you add at least one test demonstrating the PR?
• Did you delete any extraneous printlns/debugging code?
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Type of Improvement

• API modification
• Bugfix

Desired Merge Strategy

• Rebase: You will rebase the PR onto master and it will be merged with a merge commit.

Release Notes

The message passed to stop is no longer ignored. The construct was extended to accept Printable.

Reviewer Checklist (only modified by reviewer)

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[jackkoenig]

LGTM, a couple of minor comments, and don't forget to run sbt fmt to run formatting.

[jackkoenig]

If you don't mind adding back def apply(message: String)(implicit sourceInfo: SourceInfo): Stop, then we can backport this to 6.x and it's also probably better to overload rather than rely on the implicit conversion from String to Printable anyway.