In Artifactory version 3.x you could create a user plugin with CancelException and the message appeared in the UI. In Artifactory version 4.x the message send to see the logs.
Steps to reproduce:
1. Created a user plugin with: throw new CancelException("The file was rejected to deploy",403)
2. Used this user plugin with Artifactory version 3.9.1 and Artifactory 4.1.3
3. In Artifactory 3.9.1 the error message was:
Cannot deploy file 'MyPackage.1.0.0-Snapshot.nupkg'. org.artifactory.exception.CancelException: The file was rejected to deploy.
4. In the Artifactory 4.1.3 the message was:
Failed to deploy file:MyPackage.1.0.0-Snapshot.nupkg to Repository: nuget-local. Please check the log file for more details