fix #4491: adding a more explicit shutdown exception

[shawkins]

Description

Fixes #4491 - since this has come up a couple of times we probably need a more explicit message for a RejectedExecutionException from down in okhttp. Assuming #4430 is committed, this change only needs applied to a single okhttp method.

It's also possible to handle this more generally with a proxied httpclient with close checking, but I don't think that is needed as the other clients either already do that check or provide a better exception.

Type of change

• Bug fix (non-breaking change which fixes an issue)
• Feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to change
• Chore (non-breaking change which doesn't affect codebase;
  test, version modification, documentation, etc.)

Checklist

• Code contributed by me aligns with current project license: Apache 2.0
• I Added CHANGELOG entry regarding this change
• I have implemented unit tests to cover my changes
• I have added/updated the javadocs and other documentation accordingly
• No new bugs, code smells, etc. in SonarCloud report
• I tested my code in Kubernetes
• I tested my code in OpenShift

[sonarqubecloud]

Kudos, SonarCloud Quality Gate passed!   

0 Bugs
0 Vulnerabilities
0 Security Hotspots
0 Code Smells

33.3% Coverage
0.0% Duplication