Only return a failure exit code when all repos have failed

When even one repository fails in a scala steward run the workspace is not persisted by GitHub Actions because GH Actions won't persist the workspace of a failing job. Persisting workspace is crucial for respecting `pullRequests.frequency` configuration because the workspace is how scala steward records that it has opened a pull request. This means that one failing repository can cause a lot of user annoyance because their configuration is no longer respected and they get too many PRs opened.
This change aims to fix that by only returning a failure code if *all* repos have failed.
So long as at least one repository succeeded the exit code will be success and the workspace will persist. If administrators need to know what repos are failing they can use the jobs summary introduced by: scala-steward-org#3071

See more here: guardian/scala-steward-public-repos#60

Co-authored-by: Roberto Tyley <[email protected]>

modules/core/src/main/scala/org/scalasteward/core/application/RunResults.scala

@@ -23,7 +23,7 @@ import org.scalasteward.core.data.Repo
 case class RunResults(results: List[Either[(Repo, Throwable), Repo]]) {
   val (reposWithFailures, successRepos) = results.separate
 
-  val exitCode: ExitCode = if (reposWithFailures.isEmpty) ExitCode.Success else ExitCode.Error
+  val exitCode: ExitCode = if (successRepos.nonEmpty) ExitCode.Success else ExitCode.Error
 
   val markdownSummary: String = {
     val failuresSummaryOpt = Option.when(reposWithFailures.nonEmpty) {