Improve traceability of plugin dependency exception by printing

[kwin]

dependency chain

This closes #302

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[timja]

No test updates required?

Can you show actual output and testing instruction for a one-off test?

[kwin]

You can now check out with java -jar plugin-management-cli/target/jenkins-plugin-manager-2.8.1-SNAPSHOT.jar --plugins nabaztag:1.9 -d plugins/ --war ~/Downloads/jenkins.war --credentials abcd:acd:acd and it exposes now

Multiple plugin prerequisites not met:
Plugin nabaztag:1.9 Could not resolve dependencies: null

[timja]

You can now check out with java -jar plugin-management-cli/target/jenkins-plugin-manager-2.8.1-SNAPSHOT.jar --plugins nabaztag:1.9 -d plugins/ --war ~/Downloads/jenkins.war --credentials abcd:acd:acd and it exposes now

Multiple plugin prerequisites not met:
Plugin nabaztag:1.9 Could not resolve dependencies: null

Is this correct?

Multiple plugin prerequisites not met

isn't that only one?

[kwin]

Yes, but this behaviour has not changed by this PR. I will try to improve the message.