Do not throw errors up the call stack when addAction fails.
Generally this is not crucial and it is better to just continue.
From GitStepTest, an unexplained I/O failure adding WorkspaceActionImpl:
… hudson.model.Executor run
SEVERE: Executor threw an exception
java.io.IOError: java.io.IOException: Unable to delete /…/jobs/demo/builds/…/workflow/3.xml
	at org.jenkinsci.plugins.workflow.graph.FlowNode$1.persist(FlowNode.java:242)
	at org.jenkinsci.plugins.workflow.graph.FlowNode$1.add(FlowNode.java:216)
	at org.jenkinsci.plugins.workflow.graph.FlowNode$1.add(FlowNode.java:207)
	at java.util.AbstractList.add(AbstractList.java:108)
	at hudson.model.Actionable.addAction(Actionable.java:122)
	at org.jenkinsci.plugins.workflow.support.steps.ExecutorStepExecution$PlaceholderTask$PlaceholderExecutable.run(ExecutorStepExecution.java:403)
	at hudson.model.ResourceController.execute(ResourceController.java:89)
	at hudson.model.Executor.run(Executor.java:240)
Caused by: java.io.IOException: Unable to delete /…/jobs/demo/builds/…/workflow/3.xml
	at hudson.util.AtomicFileWriter.commit(AtomicFileWriter.java:112)
	at hudson.XmlFile.write(XmlFile.java:179)
	at org.jenkinsci.plugins.workflow.support.storage.SimpleXStreamFlowNodeStorage.saveActions(SimpleXStreamFlowNodeStorage.java:106)
	at org.jenkinsci.plugins.workflow.cps.CpsFlowExecution.saveActions(CpsFlowExecution.java:626)
	at org.jenkinsci.plugins.workflow.graph.FlowNode$1.persist(FlowNode.java:240)
	... 7 more