[Feature]: Validate exclusion regexes on backend

[ikreymer]

What change would you like to see?

Although frontend mostly has validation, we should also validate regex entries for exclusions on the backend, to ensure that are no invalid regexes in the exclude or include values in seed scope.

Context

If an invalid regex is entered, it can cause the crawler to crash immediately, w/o good error message for the user at the moment. Best solution is we ensure no invalid regexes are added to the crawl workflow.