For the sake of simplifying the reproduction steps, the simple Pipeline is used.

1. Create an empty git repo
2. Configure it as a implicitly loaded global shared library
3. Create a Pipeline:

 
properties([disableConcurrentBuilds()])

node {
  echo 'Hello World'
}

1. Run the Pipeline and see it fail due to 
   ERROR: Library library expected to contain at least one of src or vars directories
2. Add a sayHello step to the shared library https://jenkins.io/doc/book/pipeline/shared-libraries/#defining-custom-steps
3. Run the Pipeline again

Actual: Build fails with:

java.io.IOException: hello-world #1 did not yet start

Full stack trace is attached.

Expected: Either success or a more readable error.