When I search for the term 'ActivityNotFoundException' on SharePoint StackExchange I get no results: https://sharepoint.stackexchange.com/search?q=ActivityNotFoundException.
If I search for 'Microsoft.Workflow.Client.ActivityNotFoundException' I will get 3 results: https://sharepoint.stackexchange.com/search?q=Microsoft.Workflow.Client.ActivityNotFoundException.
This issue has already been discussed on Search within code block.
So, to summarize, some users will post "Exception has occurred" while other users will post "System.Exception has occurred". So in order to properly search through SE you always need to search for both variants.