add log message for failing validation via exception path

Currently, the catch block of free function validate() in collection_validation.cpp only adds a string error message to the results returned to the command; it doesn't log a message to the system log. All other codepaths for validate log a message at the command's conclusion stating whether validate found corruption or not.
This type of failure is affecting WT-6538 since it makes it difficult to determine which validate command in the test generated the failure.