Add crawl errors incrementally during crawl

[tw4l]

Fixes #1558

• Adds crawl errors to database incrementally during crawl rather than after crawl completes
• Simplifies crawl /errors API endpoint to always return errors from database

I've manually tested that errors are getting picked up and exposed by the API while crawl is running in local Browsertrix deployment.

[ikreymer]

Great, A nice simplification to the infrastructure - for once we're removing complexity instead of adding :)