LintDiff throws AutoRest exception when default tag is not defined in README; error is too general and Avocado does not report failure

[Konrad Jamrozik]

Details here:

https://github.com/Azure/azure-rest-api-specs-pr/pull/17975#issuecomment-2101196466

[Konrad Jamrozik]

Another instance:

• Teams chat with @bdefoy
• https://github.com/Azure/azure-rest-api-specs-pr/pull/18436/checks?check_run_id=25808213191